"This is a purple color."
Translation:זה צבע סגול.
From what I've read in other threads, זה צבע סגול means the color itself is purple (as opposed to magenta for example).
זה בצבע סגול. This would mean that an item has (or is with / ב) the color purple.
A nebulous difference, I know. That is, if I'm right. Please correct me if I'm wrong!