Personally, I don't have any problem with illustration. If I were to write a novel, I'd probably add a couple of illustrations here and there if I had the luxury of time to do so.
As for wether b/w or grayscale or colored, well........I think that is totally on your preference and which option would really compliment the tone or vibe of your story
If you were doing a detective story or maybe horror, you could probably go for b/w but if u're doing something less serious, maybe comedy then colored would be a better option. At least in my opinion. At the end of the day, it all comes down to what you prefer, what suits you best.
