Actually, this is more awesome than fun and I'm a little embarrassed that I've never shared this before. I use this website ALL THE TIME.
This website has ALL of Stampin' Up!'s current colours (current and retired) and will match colours together for you.
For example. Let's say you have a Whisper White Card Base and some DSP with the colours of Blackberry Bliss and Crushed Curry in it. What colour ribbon should you use with on the card?
Simply go to the Colorlab site and click on Whisper White, Blackberry Bliss and Crushed Curry and click "search".
The color lab will come back and tell you that you can use Smoky Slate and Rich Razzleberry or Bermuda Bay and Watermelon Wonder or Basic Black.
This tool is "the bomb" in my opinion. I hope you have fun coming up with new and creative colour combinations!