Either it would be overpowered or so subtle as to be pointless.
Regardless of how effective it is, as a mara or goon I would constantly hit turrets, teslas and reactor to build weapon resistance, and then just avoid flamers unless I was on full health.
Don't get me wrong, I *really* like this idea for basilisks (possibly dretches as well) but goons and marauders are already very effective at taking lots of damage then jumping away to heal in safety. What this would do is make players that are already good even harder to kill without having much effect on weaker players.
Basilisks on the other hand can't escape so easily and it would make a currently underpowered class more interesting and challenging to play. If anything, I'd make the resistance build a lot faster than what you propose and cap it at 80%.
I like the idea of tinting the colour as well, again this suits the basilisk because their default colour is so pale.
There'd be 3 classes of weapon resistance:
Impact: rifle / shotgun / chaingun / mass driver / turrets / grenades
Energy: lasgun / pulse rifle / luci / teslas / reactor / painsaw? / blaster?
Flame: flamer
The Red / Green / Blue components of the color would reflect resistance to Impact / Flame / Energy resistance respectively, so that the colour code would end up like this:
White - resists none
Yellow - resists energy
Red - resists energy and flamer
Magenta - resists flamer
Blue - resists impact and flamer
Cyan - resists impact
Green - resists impact and energy
Black - resists all
To make things even more interesting, you could retain some of the resistance that you built as a basilisk after you evolve to a higher class, but lose all resistance when you die. So it would really pay off to play a basilisk. Humans would avoid shooting pale basilisks unless they were sure of a kill, because an escaped basilisk now means a tougher goon later. On the other hand, humans would go out of their way to kill dark basilisks for the same reason.
Told you I liked the idea!

I think it is worthy of a mod, at least.