Golden Artist Colors makes great acrylics and a huge variety of mediums to use with them -- creating nearly limitless possibilities with their paints. They also produce the Williamsburg hand made oil colors.
Now they have come up with a very handy tool for any painter. They have a webtool called GOLDEN MXR where you can mix paints and see the results on line prior to using the paints themselves. Check it out at: GOLDEN MXR