This is a really really good website that talks about the design element of balance. It not only gives good definitions about the different types of balance, it also provides very good pictures that clearly demonstrate what balance is and how it is used in art as well as graphic design.
This site gave some great additional insights into how balance can be used in design. It was nice to get another perspective in addition to the book. It explained it effectively and in simpler terms. Definitely worth the read!
This quick video gives a very good explanation of how additive and subtractive color systems work. It provides a great example of both of them in action.