Description: Likely published in 80s to early 90s. Educational content about prismatic light and rainbow colors. Illustrated story book. I remember it being hard cover, but may have been published both ways.
World is grey and wizard/magician/sorcerer brings prism to introduce light and color to the people. Takes children characters (more than one) on the journey to discover the colors of the rainbow throughout the world which may be better defined as a kingdom. I think there was a king and a castle involved. 
I’ve located a similar book titled The Color Wizard by Barbara Brenner. It is very close to the story line, where there is a magical person painting the colors of the rainbow. In the book I’m searching for there are children and a prism is involved in the story. I don’t remember the magical one inhabiting the castle, but as a visitor. 
Thank you to anyone who is able to contribute to finding this!