This is a pretty common concern and something we deal with all the time. This applies to people of color and people who tan a lot;
Your skin is made up of many different layers and tattoos are done several layers deep. (this is why they are permanent) The darker your skin tone is, the darker the pigments will look underneath. So if you are naturally dark, or if you tan a lot, the tattoo will appear darker than if you have a pale complexion.
For example; something like bright yellow (like tweetie bird) will look a few shades darker depending on your complexion. If you are fairly dark complected, bright yellow might look more like the yellow color of a school bus.
This phenomenon is more noticeable in the lighter/brighter colors like white, yellows, baby blues, pinks etc. In darker colors like blue, reds, greens, browns, purples etc. this effect will be less noticeable.
I hope this answers your question adequately. Thanks,
-Ed