Fun fact, Andrzej Sapkowski only specified a few characters' skin color when it was important to note that they were from the same region as Geralt of Rivia. Otherwise, the only big note is that Zerrikanians have sun-blackened skin (as in deep black like you'd find in many parts of central Africa). Everything else is basically just commentary on hair styles and especially hair dyes. Very little time is spent in his books discussing natural hair colors or skin color with the exception of people very far to the north, Geralt, and the Zerrikanians. In interviews back before the video games were made, (I'm going to paraphrase here), he basically said that the skin colors and hair colors were as diverse if not more diverse than Earth and because of the ease of travel for anyone aided by magic users, you could expect to find tons of diversities in any of the cities and even in large towns. As for the magic users and witchers, they'd be as diverse as the world itself as they were trained from all parts of the world.