The Grip Flex brand line was discontinued by Akzo Nobel in December of 2013. There may still be supplies of the paint left in some places, assuming you can get reducers for it.
The Akzo replacement is called Grip Gard BC translucent paint. The toners are actually the same as their regular Grip Gard paints, but by adding a translucent binder they become plastic sign paints. The advantage is supposed to be that you only need one set of colors for a mixing station and can use the paint for both cabinets and faces, depending on the the binder you add.
http://www.signfinishes.com/NewsWeb/Documents/BC%20Translucent%20Sell%20Sheet.pdf
There is also the translucent sign paint still made by Spraylat, Lacryl 400. Spraylat is now owned by Matthews.
I haven't used the Grip Gard translucent paints. I would like to know how they compare to Lacryl.
Here is a tiny video showing a painter applying the first two coats of translucent sign paint to a piece of plastic. The first coat should resemble a clear coat that has just a little pigment in it. Build up the color with successive coats till you have a nice translucent color. If you put fluorescent lamps behind the plastic while you paint you can gauge the coverage. Back up all colors with white as the final coat.
[video=youtube;-G8dx3_dhQU]https://www.youtube.com/watch?v=-G8dx3_dhQU[/video]
Brad in Kansas City