Cue the "Sound of Music" soundtrack! There are just some products/books/techniques out there that I love so much- or use so often- that I have to share them. So my first "Fav" is Pinata inks, which I referenced earlier. Made by Jacquard, they are a solvent-based ink. (Another brand is Pantone, but I don't like them as well...)
Why are they so fabulous? Well, first of all, the colors are VIBRANT and we love that. Secondly, they are potant, and one drop goes a long way (which makes it a great deal, and we love that.) And thirdly, they work on non-porous surfaces, like glass, leather, plastic (i.e, shrinky dinks), vellum, and metal and WE LOVE THAT!!! They come in a little dropper bottle, usually for 4 bucks are so. But like I said, a little goes a long way.
The way I like to use them is to take one of your kid's wooden blocks (or hit a garage sale) and attach the "hook" part of hook and loop tape. Then cut a small swath of cotten batting and stick it on the hook tape. Dab a color, or two, or three... and you're set. It is glorious on glossy cardstock, where you can "smoosh" the inks around, blending them a little. (NOTE: not so good on non-coated papers, which just absorb the inks too quickly.) You can also just drop the colors onto the glossy papers, or vellum or transparancies, and then smoosh them around w/ the blending block. Add a little Krylon leafing pen, now you're making granite textures... yum... This is the best stuff to use on plastics, and I highly recommend buying the starter pack. You can thin/extend them with rubbing alcohol- also useful for clean-up, but they lose a bit of the gloss. The "claro" extender keeps the inks vibrant and shiny. Ok, enough gushing... but they are wonderful treat yourself to a few, and let me know what you did with them.