So a lot of times people come wishing for tattoos and colours that aren't going to work out the way they imagine. Mostly because of biology. Ink sits below your skin so your own natural tone is always going to be on top.
I really have no issues tattooing anyone - but the conversation is sometimes a bit hard to have because people always have a very specific image of what they want and they rarely take their skin tone into account.
Here is an example of the exact same ink colours but on two very different complections.
Hopefully a picture will be easier to understand than my coloured nylons over paper analogy.