Like Marah I use bare minerals and meow cosmetics. If you go to a bare minerals store or even the bare minerals counter in a department store, ask for samples of a few of the foundation shades (they can make you some sample containers that will hold 1-2 applications) and go home and apply and view them in various lighting situations. That's how I determined my color. When I initally went, they tried to tell me I was the color deepest deep....which was WAY to dark on me....like 2-3 obvious shades too dark. They also suggested colors with the wrong undertone as well. When I matched myself I home, the golden dark was a PERFECT match.
Also, I find in the summer I have to mix two colors to match my skin because I get redder with sun exposure. So my summer color is a 2:1 mix of golden dark and medium deep.
When my bare minerals runs out, I'm switching over to Meow cosmetics exclusivly. They have a powder primer for oily skin that is AMAZING (just ordered a full size of it) and I LOVE thier eyeshadows! The prices are very reasonable and you get a LOT of product for the price. I'd suggest ordering a few samples from them (samples are $1 a bag but you get 3-4 applications per bag) and trying a few and see which brand you like better and then purchase full size products.