I've added bleach and it helps, generally about a cap full per gallon. It has never affected the color noticeably, thought it's almost always slop paint so not generally a worry for us. Scented bleach helps tone down the bleach smell.
For the "some other material" I've had pretty good luck with this stuff. They also have different thicknesses available. Make a full size print of the design you want, lay this stuff over and with a *very* sharp exacto knife trace it out. Goes must faster and the stencil lasts longer than many...