From different teachers I have heard many times in different ways - value does all the work and color gets all the credit.
On her blog Sue Williams talked about a tool she uses and it is a great idea. Here is a link to her blog. Please take a look at her paintings, too. The first shown called "Arrived" really uses shapes and values for its success. It just twinkles.
Thank you, Sue, I made this tool immediately. I like the size of yours better, but I had this scrap of wood leftover from a painting. Next time I will make a bigger one.
While you are painting if you have a question about the value of your choice paint right on the board to check your value .
I did a lot of thinking about value in this painting. I like these two colors together.
Note: Patti Mollica's book Color Theory covers this topic very well. Sue learned about this from one of Patti's videos from YouTube as she mentions on her blog. I recommend Patti's book and any of her DVD's. She is also a master of very strong brushwork, too. The book is concise and packed with information, one of my favorites.