Silhouette stamps are perfect for adding multiple colours at the same time, and a simple way to do that is by using water based markers. Place your stamp on an acrylic block and then colour each area with a marker as though you're colouring an image. Make sure you hit every part of the stamp with colour - placing a piece of white paper underneath your work will help you see where you've placed the ink.
I like to mix colours directly on the stamp. Generally, I start with the lightest colour and then tap or stroke in the darker colour to whatever area I like. There's no need to fuss a lot with this technique, just enjoy!
It doesn't take too long to build up a healthy stack of coloured images ready to be made into cards.