2527 Tulip 29c Red Missing?

Just found this pair in some scrap postage.

My Scott Specialized catalogue from 2021 does not seem to list either a yellow-only Tulip 29c (design A1879, booklet catalogue number 2527) *or* a red-missing error. Or any colors omitted errors, for that matter.

Ideas, anyone?

100% typical of sunlight fading. The red end of the visible spectrum fades-out first. See this:
https://stampsmarter.org/learning/A...gelings.html

Mr. Becker meant to say, ... sunlight or office fluorescent lighting or similar ...
People often forget various settings cause fading of inks like red or yellow... depending on light source and distance (strength), is the time it takes for all of that color to disappear.
There are many references in hobby literature about post office lobby displays in glass cabinets with fluorescent lighting that faded some colors of the opened stamp booklets etc displayed (I doubt such displays are still around, that would be interesting posts if anyone here is in some of the largest cities with post offices that still might have one or two such cabinets? or even wall mounted displays possible, too.