This would be very useful - separating character scenes, themes and so on. I do it in the real world, pink and blue, and it would be nice to do it here too.
I agree, but I think that feature is not, umm, in the cards for the moment.
For the sake of a cleaner, consistent interface, Keith has decided to let the colour of the pins --rather than the cards themselves --perform this function.
Cheers,
E
But you may assign colors to the labels for each card, along with your own names for the labels, and they show in the Outliner, displayed in a list-table format. In Corkboard, label colors also show in the third pane.
Can I just add a +1 for this feature? Nice pastel colored index cards would really be a plus. 
Andy
You can add as many "+1"s as you want, but it won’t change before 2.0. As of 1.01, you can choose the background colour of your cards, but this is for all of the cards, not for individual cards. As AmberV has pointed out, the pins of the cards displays the label colour. Like I say, this will probably be something I look at in 2.0, but it is unlikely to change before.
Thanks for the suggestion,
Best,
Keith