Randy, I don't know who thought up the ws-washer fluid idea, but it works. I'm fairly certain that whoever thought it up is a lot smarter than I, because it would never have occurred to me to try something like that
About your question: there's really only one way you can go: acrylics. Floquil/Testors discontinued the Polly S line a few years back, and all you can get now is PollyScale acrylic. But here's another tip: most of the various brands of "craft" paints (available at Wal Mart, Michaels, etc.) are also acrylics, and many of them have shades that are exact (or nearly exact) matches for the colors in the PollyScale line. Look for brands such as Apple Barrel, Plaid, Delta Ceramcoat, and American Folkart. Most of them are very inexpensive, ranging from around $0.44 - $1.00 per bottle, and they can be brushed or airbrushed just like Polly.
I have found that the cheaper brands (Apple Barrel and Plaid, $.44 at Wal Mart) are good for brushing, but the more expensive brands (Ceramcoat, for example) have smaller pigments and are easier to airbrush smoothly.
They also can be thinned with distilled water or alcohol to the consistency of wood stain and then be used to stain stripwood. But that's a story for another time...

(If you want to see the results of stripwood staining with thinned acrylics, look at the machine shop photos on my website; all of that wood was soaked with an acrylic stain.)
I forgot I can post a photo with this. I'll see if I have a machine shop pic that's small enough to post with this. If not, check the website.