If you are changing from a light color to a dark color you may be able to just scrape all the existing plastisol off the screen and then wipe it down real good with a towel. Then print onto a used t-shirt with a clean squeegee a couple of time to push and final residue from the screen. Load the screen up with your new color and run a hand ful of test prints until all evidence of the other color is gone. This isn't ideal...but it may work for you in a pinch?