Believe it or not, that was one of the things I had to consider when I embarked on my Mystic Valley series. What color does a woman turn if she blushes? What color does he turn if he's angry?
When a character is a different color, every emotion is exhibited in a completely different range of shades from what we are familiar with. Body parts that are normally darker--such as nipples--cannot be described as pink or cinnamon or strawberry because that just doesn't compute when the rest of the character's skin is some shade of blue.
As the author, I had to get creative. Blushing was more of a lavender shade. If someone went "white" in shock, it was icy blue. And, ahem, body parts were various shades from light blue to midnight depending on the skin shade on the rest of the body.
I once read a book with green people. I noticed that the author had a similar problem. So that's always a consideration when I think about characters that have other than normal human skin tones.
I also discovered (well after my series was established) that there really are blue people. Apparently a mineral deficiency of some type makes the skin blue. There's a reasonably large family in the Appalachian mountains that carries the gene. So, there really is nothing new under the sun.