As someone who is pale with cool undertones, I find foundation shopping quite difficult. I can almost always go for the lightest shade possible, but often the tone of the foundation is yellow and ends up making me look a little sickly. I have a stockpile of foundations that I like the formula of, but the undertone is all wrong.
Sephora just recently started carrying Make Up For Ever's Chromatic Mix pigments for water-based foundations and oil-based foundations. These little guys pack quite a punch and come in a variety of pigments to alter any makeup you're using: red, yellow, black, white, blue, and brown. I ordered the white and the blue pigments for water-based products because most of my foundations are water-based.
The white takes my foundation a shade or two lighter, and the blue works to counteract any yellow tones and cool the tone slightly. It took a little practice, but I've finally figured out my perfect formula to add to my foundation: 1–2 drops of blue and 3–4 drops of white. I hope you can tell from the images above, but the foundation on my palette is lighter and less yellow than the product on the end of the foundation tube.
These little droppers are serious miracle workers (and at $14 a bottle, they could potentially save you from spending a lot more testing foundations that don't work). Bonus: I get to feel like an artist when I do my foundation as I'm mixing the pigments in with the base! (You don't want to see my lack of true artistic abilities when it comes to drawing, painting, etc.)