Colour is a good starting point methinks. The main two things to think about colour-wise when you’re trying to find a foundation is 1) the lightness of your skin and 2) the undertone of your skin.
When it comes to picking how light or dark your foundation should be, it's good to test it on your face and not your hand. I always think I know my skin tone well enough to get away with not testing it properly and I’m always wrong. *Sigh* I just don’t learn. Generally the best way to test foundation is on the back of the jaw and coming down onto the neck. The foundation shouldn’t just match a certain area of your face, it should ideally match your whole neck and chest too.
If you’ve tested a bunch of colours and you think that you’re actually in between a couple of them I would suggest going for the lighter one. I know if can be tempting to go for the darker one, but there are a couple of reasons to go light: firstly a lot of foundations oxidize. Once they’ve been on your skin for a little while the shade will deepen a touch. Secondly, there’s too much risk of the dreaded “mask-effect” if you go too dark with your foundation. Better to use a little bronzer if you’re feeling too pale after the foundation.
Okay, right about now would probably be a good time to explain a little more about undertone. It is definitely one of the hardest things to explain if you’ve never really considered it before. So if you take a good look at people’s skin you’ll notice they tend to appear a certain colour over-all. In general they’ll look a bit pinker, a bit more yellow/orange or neutral. So I’d first decide which one of those you fall into and then within that category you might notice you may have some other undertones thrown in there too. If you’re yellow toned, for example, you might notice you’re a bit olive too. To get the most realistic match you would ideally find a foundation with all of those subtle tones in it. It can be a bit challenging to find a foundation that truly matches because often times brands will lump you into just plain old pink (NW at MAC) or yellow/orange (NC at MAC). Sometimes it just takes trying a few different brands of foundation.
What I do quite often if I’m going to look for a high-end foundation is I first check the Sephora website. With most of the foundations now they break down the shade into “fair/light/med/dark” and what its undertones are. It’s nice to jot down which ones you think will work for you before you look at them in an actual store. Sometimes I find with the crummy lighting in most places it’s hard to tell what the undertone of the foundation you’re looking at actually is.
One of the things to watch for when you’re figuring out your undertone or getting colour matched by someone is redness/hyperpigmentation on your face. I have the same problem all the flippin’ time when people try to colour match me. People look at my irritated blotchy red face and assume I have a pink undertone, but if you look just a few inches further down at my neck you’ll see how yellow it is. If your undertone is pink and you use a yellow foundation, it’s possible to end up looking kinnndddaaa guido-esk. Isaidit. Whereas if your undertone is yellow and you use a pink foundation then you might find you end up looking a big ashy.
A final note:
...and perhaps the most helpful thing I can suggest: it has taken me so many wrongly coloured foundations to learn this lesson, but for the love of God test a foundation before you buy it (particularly if you’re going for an expensive high end foundation)! Department stores and drug stores are often quite poorly lit and it can look like you've got the right shade on when you're in there, only to find that in natural light you're wearing the wrong colour. So it’s pretty handy to take in a couple little pots, take a few colours home and try them on and check yourself out in various different locations before going back to make your purchase.