The depth of carbon penetration is controled by time at heat. If it is believed that the current part is sufficiently hard, it is only necessary to recolor at coloring temperature (say in the 500 600 F range - and this is what several of the recolorers seem to be doing). However, to re-establish hardness, one needs to heat above critical for the given steel and quench. Time above critical has no effect on hardness other than increased cabron penetration (the iron "soaks" up more carbon to form a higher carbon alloy which may be harder). If it is believed that the carbon penetration on a given piece is sufficient, a heat to critical and immediate quench will keep carbon penetration to original, re-establish the OE level of surface hardness, and offer the option of re-establishing a color layer (case color) on the surface of the part.
