I am also still curious if anyone uses airbrush colors instead of gel/paste to color their royal icing? It would be less expensive to buy just the airbrush color instead of both in each color if that were the case. If it gives the icing a pearl/metallic sheen, it might allow for piping icing decorations instead of painting them with dust in some cases. An airbrush is still beyond my budget, but it's always nice to plan ahead. I'm off to get my purple on now
I don't know the science behind fading, but the fondant fades when exposed to light and air; it's more a function of that exposure than how recently the fondant color was mixed. (I often mix fondant way in advance; some fading will happen on the outside of it, even if wrapped and stored in bins . . . but this fading happens VERY gradually compared to fondant on something exposed to air.) It may be that the egg whites in the royal icing act as a color fixative . . . but pure conjecture. And, again, it just happens with certain colors and brands of color. I've seen very little fading with my Chefmaster pinks (even non no-fade ones), yet I've had terrible results with the Wilton non no-fade pinks.
I don't use my airbrush colors in royal icing - they're really loose and mess with the consistency more than I like.