Neat little game, high learning curve though! Took a while to figure out that you can only paint the second color biomass OVER the first color biomass, and you paint the second color by dragging off of the starter pool of paint. (Or by mouse wheel in the stuff menu.)
On Wave 21, I got into a state where I stopped being able to build new units. I had 40 blue currency, 6 yellow, zero units on the map — and it wouldn't let me build anything new. So, game over!
One general thought is that the shielded fellas didn't seem all that fair, maybe, without a way to move your units. (Ignore that if there is a way to reposition your units that I missed!) I basically had to just look at 'em angrily until my units happened to wiggle into a position to hit 'em.
Still, enjoyed my time with this game. Well done!