It is a very time consuming exercise, but worth the effort - and that is to take each colour that you have and mix it with every other colour and paint little colour charts. It is most amazing to discover all the colours that you can mix.
You can go further and mix three colours, or add white, gray and black. I have done it before, but I am doing it again, because I have bought some new tube colours that I have not had previously.
Another exercise in colour would be to take pictures from magazines and try to mix colours to match specific colours.
Another good exercise is to do small paintings with a limited palette, in other words with only 2 or 3 colours plus white. You will be amazed at what you can achieve doing that.
This painting was done with only 3 colours plus white.
I am promoting the work of Richard Robinson, because he is a great painter in every aspect, but especially in the area of use of colours. You can sign up at his website and he will send you a lot of FREE information, video clips etc. You can also buy DVDs from him which is really good. Enjoy!