How to stop mushrooms from discoloring everything?
Based on comments, the likely culprit is moisture cooking out of the mushrooms and into your other ingredients.  Mushrooms contain a surprising amount of liquid, and when cooking them you'll see that they shrink down significantly due to moisture loss.  If you're adding them to other ingredients, some of the resulting liquid is hanging around in the pan long enough to discolor other, lightly colored items like eggs or tofu.

What I would suggest is sauteeing your mushrooms by themselves for at least a couple minutes, to release their moisture prior to adding everything else.  Here's a good step-by-step guide.  Be sure you salt them to draw out moisture, and make sure that you don't crowd the pan (see this question) so that the moisture can cook off effectively.  Both eggs and tofu should cook fairly quickly, so you can add them after the first couple minutes and the mushrooms can continue to brown and cook afterwards.  If you have other ingredients that need to cook for an extended amount of time, you may want to cook your mushrooms  separately if the color bothers you.  If you're using a separate pan, this won't really add additional time, but you will have extra cleaning to do.

Mushrooms are funky that way - common sense might indicate that they be cooked like vegetables, but there's a reason why Alton Brown suggests treating them like meat instead.