Yellow bases skintones tend to be more olive based (even if you are very fair) - you tend to have more sallow skintones.
Pink based fair skins are more translucent, often with freckles, may have a tendency to blush more often.
Trouble is that many of us are neutral, so slap bang in the middle.
Often you need to match your foundation to your neck (where the colour may be more true) than your face.
Some foundations (particularly the Smashbox Healthy FX) are very forgiving - but others are less so and as Shortcake say, will make you look carp - even though it seems to suit the skin colour (fair/light/med etc...)
When you find your perfect colour you don't often realise that it is the colour match that makes it look fantastic.