you can also say מה צבע הבננה? what is the color of the banana but if you want which - that's איזה
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 ><
According to your examples, it seems איזה is used to find out something, while באיזה is used to decide.
HHmm yes I think yometimes but איזה צבע אתה אוהב And באיזה צבע להשתמש Should be identicle in that sense and they are not in the use of ב
One is asking of a color already chosen, the other asking to decide on a color.
Green, then yellow-green, then yellow, then yellow with black spots, then finally black.
That's another of the occasions where I discover I don't get English prepositions. In whaf color is ok but of what color isn't?
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.