I've gone postal!!!
On the flower card - the background allover stamp was stamped in black, and the colors were added using marvy ink pads and stipling brushes. Literally - you lightly pound the brush (it's more of a tap) into the ink about 5 or 6 times, and then do the same onto the image so the ink can come off. The more you do this, the brighter the colors become. (i.e. it may take a total of 25 total taps onto the inkpad, and 25 or so onto the card.)
I used the same technique for the background on the purple Christmas cars, although a lot less, since I wanted a muted background. And I started with a wet stippling brush, which made the affect a little different than the brighter colors.