My personal opinion is that sunlight alone doesn't fade casecolors.
Handling the piece, even the slightest wear will do it. Though some pieces are prone to wear much easier from handling than others.
Just the way the individual guns were done.
I have read some reports about sunlight fading case colors on guns in display racks for long periods and not moved around. One side faded where the sun could reach it, The other side not.
I really don't know,,just my opinion.
Getting the colors to appear is not really that difficult. But there are so many variables in the works when that happens, that any number of them can be slightly changed & Colors will still appear. But then you can get what we called 'soft colors',,those that wore off very easily . Or with changes in the variables, colors that withstand the handling and wear better and don't disappear on you in a season of light use.
Some mfg'rs colors just seem to withstand time and handling better than others. Just the particular way they did theirs I guess.
Maybe the steel alloy is involved as well. Again there are dozens of variables that can be 'adjusted' and still get colors. What else is changed when the adjustments are done could be one involving how permanent they are.
Colors can be had with no surface hardening, that's another variable to the whole thing. These seem to wear easier as well.
Cyanide colors always seemed much more resilient to wear than charcoal process colors.
We never did any cyanide work (in spite of what the NYSDEC thought when they came knocking, but that's another story).
But plenty of guns came in with the cyanide hardening and colors on them to be re-done to wood/bone charcoal colors.
Cyanide hardened/colored parts seemed more difficult to anneal.
The technique used was caveman, maybe that had something to do with it !!
But it worked fine for annealing guns that were orig bone/wood charcoal colored.
Cyanide colored parts sometimes had to go through the 'process' a couple times to be assured of a soft surface before I could do any re-cutting on them.
