I'm not sure when it's correct and when not, but sometimes ב is not suitable and sometimes required (or slang/not good hebrew without) איזה יום מתאים לך? which day is good for you? באיזה יום נלך? on which day should we go איזה צבע אתה אוהב? which color do you like? באיזה צבע להשתמש? which color should I use?
idk which is used when ><
In English, we almost never say "In what color" or "of what color". We usually just say "What color is it". I think the comments above were trying to explain why the bet was used, showing that it means in or of, even though they aren't present in actual spoken English most of the time.