Refer to this topic in FAQ for more info on coloring.

As for coloring this kolam, you could draw it first using rice flour and then fill the kolam with the colored powders (should be available in stores as rangoli colors or as color powder used to celebrate holi). You can mix the color powders with the rice flour to get the desired hue.

After filling in the colors, if you want the dots to show through, just relay the dots using rice flour on top of the colors!

As for as I know, this should not stain. As a precaution, test your powders on a small area and see how it goes.