You can only talk about brightness comparison at same wavelength level. I have 450nm show lasers ranging from 500mW to 3Watt. I can tell the difference (put laser side by side) if power difference is >500mW.
In terms of color: use blue (or UV laser). It has shortest wavelength, thus higher energy than red/green.
Secondly it's more important that you get the laser beam focused at the burning point.