We know that for general graphs vertex coloring problem/ minimum clique cover problem are NP-hard and for perfect graphs, they can be solved in polynomial time. My question is that:

Are there any families of graphs that we know vertex coloring problem/ minimum clique cover problem are **NP-hard**? I appreciate if someone can give the name of some of them or any reference for them (in case they exist).

I have a problem and I want to show that it is hard and I want to find such a family of graphs and use them as an instance for my problem.