Other ways I added color are: staining (around the edges), sponging on ink, coloring with markers, computer toner, and watercolor paints. It's hard to see the painting, but I used a baby blue color over the sentiment that I printed out on watercolor paper. Then I used a circle punch to pop them out of the paper.
A challenge like this can really make you conscious of how you decorate your card with color. It was fun to step out of my comfort zone and get creative.
I hope you'll give this challenge a try and share other ways you can add color to your cards!