I started using the Australis Match Maker Shade Adjusting Drops in Lightening after buying the Body Shop equivalent for several months. The Australis one works just as well and is much more affordable! It lightens your foundation without any impact on the formula (at least with the ones I tried), and is generally a bit of a magic wand for gals with pale complexions like me!
To use this product I would take my normal amount of foundation on a brush, then add a lil bit of the Lightening drops. One drop lightened my foundation by about half a shade to a shade. If I was wanting a full on Snow White look I'd add quite a bit, about a third of the amount of foundation I was using. Then I would just apply the brush straight to my face, swirling the products together on my actual skin. This technique worked really well, and resulted in a smooth, even, lightened makeup look. However you can also mix your foundation with the drops on the back of your hand super easily too.
My only critique of this product is that the drops don't come out super easily from the dropper. It can be quite hard to get the exact amount you need, since the consistency of the product is quite thick. However it's not that hard to develop a technique for getting the right amount out.