Pigment that makes gum insoluble w/o exposure?
Chris Anerson wrote.... CUT
Anyway, on to red. I have never had a red spontaneously harden, but I have found that students will tend to confuse two things--stain and overexposure. I have no idea if either of these are your case.etc.
Chris and all... beware of speaking in colors, that is, just by color name. It's my experience that the brand of paint, as well as the particular pigment (by number) and its source (manufactured?, mined?, if so, where?, etc.) are crucial variables. Among others.
For instance, some makers include a so-called "dispersal agent" in all or some of their tube colors. In my experience the dispersal agent can very well disperse the pigment right down to China -- and you can NEVER EVER make it clear.
Besides dispersal agents, some makes of color have fillers, emulsifiers, thixotropic agents, foreign agents, real estate agents, or other "improvements" in some or all of their colors.... I suspect that just naming "orange red" or "blue red" or like that, especially from a Gum Authority such as Chris, could drive some of the uninitiated to despair if not worse (eg., platinum).
Which is to say, to be on the safe side, a brand name should probably be attached to such observations about paint behavior.