To Create an "impressionistic" brushstroke effect: This is a very simple way to add interest to your background but it *does* take some practice and a certain willingness to make mistakes.
Remember, if you *do* make a mistake and smudge off too much color, you can always start over again by simply using the alcohol ink blending solution to whisk off all the color off of your domino. The blending solution acts as an "eraser" of sorts by adding a squirt of the solution directly onto the domino and then using a rag to wipe off the ink and solution. Be careful though, as the solution can also dissolve the color on the "back" side of the domino (the side with the dots!) and if you want to keep the back side clean (for a more polished look to your finished domino) you don't want those inks to be lifted off and smudged.
Using a small, inexpensive paintbrush, squirt some of the blending solution onto the bristles. You want the bristles wet with the solution, but not dripping wet. If the brush is too wet, simply run the bristles across a dry rag once or twice. Next, swish the brush lightly over the top of the domino in a soft, circular motion, until you have acheived the desired effect.
To intensify your color, add a little bit more ink from your pen, and continue to swirl the paintbrush over the top of your domino.
Now, you can see some of the varigation of the color, but, it's become more blended and the colors appear to be painted on.
Using just a few simple tools: Sharpie Pens, Alcohol Ink Blending Solution, Q-tips and a paintbrush you can create some really unique and interesting backgrounds on your dominos.