That seems very OP to me. At first level, you could build yellow filter armor that would make somebody or something totally immune to most weapons, and which can only be destroyed by something that's impossible to get before 12th level. (Not counting wands etc.)
Really, yellow filters are so strong that I'd imagine they'd be used everywhere in a world with Grammarie, which in turn would lead to metal weapons falling out of use.
The rest of the filters are relatively okay, it's just yellow filters that are really broken. To fix this, I'd suggest changing yellow filters' vulnerability to Sonic, which is acquirable at much lower levels.