You've probably noticed when you go to the store that any highlighter you pick out will have several shades available, which can make things a little confusing. You're totally correct in your assumption that they are for different skin tones, but also, they are for different undertones. The lighter the highlighter, the better it is for a light skin tone, and the darker the highlighter the better it is for a deeper skin tone, BUT you do want to factor in your undertone as well. Cool pink highlighters work great on cool undertones (this one from MAC is great), whereas the warmer gold and coppers work great on skin with a warm undertone (like this one from L'oreal). Once you find the right highlighter shade for you, you'll notice most brands will have similar selections so it'll be easier for you to pick one out in the future.