The General Discussion forum has a recent thread about the color of concrete, and I’m currently trying to optimize the granite/gray color for rocks on my layout. Others have asked questions about colors as well.
I found an online color mixing tool that I think is the best of those I Googled. It’s fun to play around with and might make trips to buy paints more productive. It’s helping me and hopefully might help others as well:
http://jacksonsart.com/colourmixer
Rick Krall