After a bit of informal research on my own, I've concluded there are a few things or conditions that can cause the pasta machine to discolor polymer clay. This information may differ a little from what you've been told up til now.

From personal research and testing, I've concluded that there are three discoloration types:
1) light or dark grease or oil along the outer edges of the clay sheet,
2) tiny (hard or soft) bits of various colored clay deposited on various areas of the clay sheet,
3) dark to black smears or streaks on various areas of the clay sheet.

1) greasy, oily smears along the edge of the clay sheet.
Lubricants are used to keep a pasta machine's metal parts turning as smoothly as possible. Because the pasta machine is intended for food use, it unlikely the manufacturer would improperly lubricate or over lubricate it. It would be a real turn off to see one's pasta full of gear grease. However, for whatever reason if there is too much lubrication, it can migrate onto the roller surfaces and scrapers, then to your clay.

This is the easiest discoloration problem to correct. Hold a paper towel on the edge of the rollers while cranking the pasta machine to absorb the excess lubricant. Then pass light colored scrap clay through the pasta machine to see if the problem is gone.

2) Bits of various colored clay on various areas of the clay sheet.
Sometimes the bits are soft, sometimes they're hard. What they are are little return gifts that the scrapers had scraped off clay sheets as they passed by, clay that has accumulated between the rollers and the scrapers over time. That's why those little bits can be of various colors. Under certain circumstances, the clay decides to escape from the scrapers and back onto the clay.

To correct this problem: remove the scrapers and wipe away all that residue clay with a good degreaser or a thin lubricant like WD-40 or mineral oil. More on how and why removing the scrapers isn't the nightmare it used to seem - later.

3) Last but not least: dark to black smears or streaks on various areas of the clay sheet.
This is the biggest complaint of all. Most assume that these streaks are caused by the pasta machine's lubricant. Most complain about being plagued by some streaking throughout the course of using their pasta machine, no matter how old it is. Other's don't notice it. Most say it's more noticeable when their machine is new. Since I recently acquired a new Atlas and experienced a great deal of this streaking, I wondered what exactly was going on.

After a bit of research and experimentation, I have a working theory. First, I learned the rollers and scrapers of apparently all pasta machines are made of stainless steel. Of all types of steel, there are over 40 kinds of stainless steel depending on the recipe of the combination of the following materials; iron (at least 50 percent or more), carbon, chromium, nickel, molybdenum, columbium, titanium, manganese, sulphur, selenium, etc. I believe it's the loose black carbon molecules that are being leeched from the metal by some property in the plasticizer. In other words, some plasticizers can promote tarnishing; a form of corrosion.

What I have found out is steel is not invincible, not inert. Besides water, it can react to various chemicals, acids and alkalis. And I now think it reacts to the plasticizer in the polymer clay. And it appears that the greater the amount of plasticizer in the clay, the greater the reaction. The result of the reaction - that infamous dark to black streaking - tarnishing.

I noticed because the clay that seems to cause my pasta machine the greatest streaking problem is plain ole soft translucent and it has the highest ratio of plasticizer per weight/volume.

My current hypothesis is when a sheet of clay is passed between the rollers, sometimes a small amount of clay is scraped off the rollers and accumulates on the scrapers and it sits there, reacting. Minute traces of plasticizer sits on the rollers, also slowly reacting with the steel.

The reason I think some have more problems with streaking than others? It depends on the exact composition of the steel in a particular pasta machine's rollers and scrapers, the kinds of clay processed through the machine (lots of soft and/or translucent clay), and how much stress/pressure the rollers and scrapers are subjected to. Increased stress facilitates the release of the accumulated clay hiding inside the scrapers.

Another possible factor which can promote tarnishing? Acids. Acids are known to turn carbon steel black. It's possible your own body chemistry hastens tarnishing of the pasta machine rollers if your body chemistry is somewhat acidic and you handle to rollers often. But that's just a guess on my part.

I'm still researching the hows and whats about stainless steel, corrosion and how to reduce or prevent it.

You're probably just thinking, "Stop! I just want to know how to get rid of those pesky streaks?" I went into the small amount of detail above because it may help in how you approach the care and maintenance of your pasta machine.

Like fine cutlery for the kitchen, I think pasta machines require a bit of care and consideration, probably a bit more than you thought. In general, if you're having streaking problems, you'll need to remember to clean away as much of the plasticizer and clay as you can. It's like remembering to not let your expensive steel carbon knives sit in water or stay wet for too long.

The best solutions for cleaning plasticizer residue and clay from your pasta machine include isopropyl rubbing alcohol (+90% by volume) or mild household "orange oil" based cleaners/degreasers. To clean and reduce tarnishing (a form of corrosion) of those steel rollers and scrapers, use penetrating lubricants like WD-40 or CRC. Handle the last two carefully and wipe off all excess.

By all means, don't use anything that is considered a moisturizer, anything that contains glycerine or any other emulsifier. Moisturizers and glycerine can be thought of as thick water. It coats like a grease or cream and it contains lots of water. While it is great for moisturizing skin, it'll linger on the metal and cause metals like steel and iron to rust. Certain brands of baby and hand wipes contain glycerine and other moisturizers. Keep those away from your pasta machine.

Now the good news is if you've got a common household pasta machine like an Italian Pasta Queen or a Mercato Atlas (AA), it's quite easy to take the machine apart to thoroughly clean the scrapers as well as the rollers and lubricate where necessary - what I call a "2nd stage cleaning and lubing". (Third stage cleaning is complete disassembly of the pasta machine, to be covered at another time.) Though not impossible, Imperias and Trattorinas are a bit more of a challenge to take apart for a 2nd stage cleaning and lubing and is not covered here.

On the crank (handle) side of the pasta machine, use the small point Phillips head screwdriver to remove the small screw that holds the outside half of the side panel. Remove the outside half of the side panel.

Flip the pasta machine so you can access the bottom. Use the medium point Phillips on the two screws of one foot to remove the crank-side foot.

Flip your machine so it's upright again. Use the 10mm socket wrench to remove the two nuts that attach the inside half side panel. Be careful, the inside panel may have sharp edges. Grip the panel then wiggle, rock, cajole it to pull it off.

The panel held several things in place; the pair of smooth rollers, a pair of scrapers, a pair of bracing rods, and a pair of fenders.

Toss those fenders aside. They're history! You'll never need them again.

Get ready to pull out one of the scrapers. Before doing that however, carefully note their position and their insertion points into the other side panel. This observation is very important!

Now, pull just one scraper completely out and examine the edge. You may notice a buildup of clay. Use a paper towel lightly soaked with your choice of cleaner, degreaser or lubricant to soften and wipe all the clay away. Then wipe the scraper clean with a clean, dry paper towel. Put that scraper back in its slot.

Pull out the other scraper and clean it the same way as the first scraper. Return the second scraper to its respective slot. The major cleaning task is done! If necessary and you have some silicon grease you can take this opportunity to lightly grease the ends of the rollers. Take care to not over grease, otherwise the excess can end up on your clay.

Note: Positioning the scraper blades is very important to a successful outcome of using pasta machine. Without them, the clay would stay on the rollers and keep rolling around, building up layer after layer.

Once the rollers have done their work, the scraper blades guide the clay away from the rollers. The scraper blade edges needs to remain smooth and close to the rollers.

If you jam tools into the space to remove scraped clay, or if you add bits of metal or sand, etc, in the clay and pass it through the pasta machine or if you force clay pieces between the rollers that are too thick, you could damage the scraper blade edges or even bend the blades, making them annoyingly ineffective. You'd need to replace them

Now let's put it back together.

Position the inside side panel so that its position matches its counterpart on the other side of the pasta machine. Align it so that the rollers begin to slide into their openings. Next, align the bracing rods so they can begin to slip into their openings. Finally, align the scrapers so their tabs are directly in front of their slot openings in the inside side panel. Press the panel until it kinda clicks into place. Replace the nuts on the bracing rods.

Flip the entire machine so the bottom is up. Replace the foot, screw in those two little screws.

Flip the machine so it is upright once again. Replace the outside side panel and put its screw back in.

Slip a clean dry paper towel down between the rollers and with the handle inserted, turn the rollers but don't let the paper towel pass through. Crank clockwise and counter clockwise. Finally, pass a piece of light colored scrap clay through the rollers a few times. Check the clay to make sure it's clean. You're done. When you've got the right tools and you've done this once or twice, this entire procedure takes about 5-10 minutes!

These procedures should help reduce most of the reasons for discoloration of your clay by your pasta machine and keep your pasta machine in fine working order for years to come.