I like white pies with SOME color mixed in. What is the best way to do this?

Add pudding? What about blueberry/cherry pie filling?

In the past I've used a very small amount of food coloring and just made sure to mix it in very well so that it was diluted sufficiently not to stain.

Food coloring works well. I usually use a five gallon bucket and dump all my cool whip in it. Then a drill with a drywall mud paddle to smooth the mixture out. I would add a few drops of food coloring before mixing; it's easier to add more coloring than to take out...

What color are you looking for? Jello can be used for some stuff. A slight yellow? That's vanilla. Brown? Brace yourselves...chocolate. (Amazing, right?) Green can be lime jello or pistachio pudding. A slight red color? Strawberry. A stronger red? Cherry. Purple? Grape. Blue? Raspberry (go figure).

You can also use kool-aid on >certain< substances. However, be careful because it can stain skin (temporarily), and oddly enough, it can't be used with shaving cream. Makes the shaving cream dissolve quicker. Not sure why, though, since pudding and jello mixes don't.

Has anyone used pie filling?

I'm a fan of using actual fruit fillings with the cream. If you live in the US, it isn't expensive; As you noted, just pick up a can of blueberry, cherry, etc. pie filling. I think it looks way better than pastel-colored cream.