回复：Not sure about this.

I am not sure lining (friction material) thickness can be checked by color. But I know some lining has "underlayer", which is not designed for braking, but for thermal isolation. If lining has worn to underlayer, for sure pads must be changed ASAP. BTW, lining wear is not even. most cases inner pad wears faster than outer pad, and leading end of a pad wears more than trailing end. The best way is to check pad condition by taking off the wheel.