Since case-hardening colors are so closely related to temper heat colors, localized heating can be used to alter the the colors or to add colors. A natural gas or propane and oxygen torch also works very well when the flame is very small and oxygen rich. These methods have been found to be very useful in obtaining excellent colors and consistent repeatable results.
The colors of Case-Hardening are produced during the "Hardening" (quench) proces. Temper colors are produced by heating to a degree which removes hardness. "Spot" tempering a hardened part results in a smorgasboard of hard & soft spots. This Method should "Never Ever" be applied to a gun frame.
Loss of color does not result in loss of the hard case, tempering it does. The hard "Case" may not be necessary in order to contain the forces of firing, but it "Was" put there for a purpose. It definitely should be kept uniform & not sporadic.
I do not know what method "ED" uses, but the appearence to me is of the process where the part is "Warmed" (less than the heat of boiling water), not to a temper heat) & then daubed with chemicals. "IF" this be the case this does not affect the hardness of the case at all, only the color of the surface.
Personally on my own guns even if the color is virtually all gone I just leave them "As-Is". This "Faux-Finish" though is rather easily applied & does not require a "Master" nor is it worthy of a "Premium" price.