I have used food coloring in water. Not super bright and vibrant, but quick and easy and cheap. Sand the project, dunk for a bit, then hang to dry, finish with clear spray acrylic (doesn't yellow like some things) I have found that the raising of the grain is minimal but you might want to test it out.
I have also had really good luck with using permanent markers. I cut some small wordart that I made into pins and needed it to be bright red. It was too late to drive and get spray paint, and I did't want hand paint all of them. I tried a red sharpie, and the result was bright and awesome.