Have you ever noticed that we as humans associate colors with feelings and themes? For example when I say the word "rich" in western culture what color do most people think of? Green
What if I said "love"? Red?
Knowing the meanings behind colors is something that comes in handy as a designer. Knowing that white in the western world is reserved for wedding clothes, but in the east, it is worn by widows or by people in mourning.
For a brief overview of colors' meanings both east and west check out: Incredible Art Department. Their descriptions are a good overview of color meaning and a good jumping off point for further study.
When researching a play, knowing different color psychology meanings can help subconsciously get your themes across to the audience. Books such as, Green: The History of a Color give in depth looks at colors in western cultures. The same author also published books on Blue and Black.
Other reference book options:
If color psychology intrigues you, but books seem a bit too much for you at the moment and you just want color your feelings, you can always use this pencil set. I hope this gets you thinking about the meanings behind your color choices on and off stage.
Color has always been something that influenced me as a designer and a blogger. Here's another color theory themed article! In it I give a few specific examples of playing with and against color theory on stage.