I am a big fan of Google Calendar and I use it relentlessly in my implementation of GTD. One of my "prepare for a trip" steps is to print out the up-coming week (so I can refer to it when I am on a plane and not connected).
Tonight, as I am preparing for my upcoming trip, I hit the printer icon and the preview looked different. It was in color!
As you can see, each calendar is in a different color. It is a nice feature and a long time coming! Well done, Google.