Missing colors can be a hard thing to pin down. Especially red since red is very prone to fading. Red was also often done with two layers, one pink, the other the bright red.
Here's a few examples of different missing color issues.
The top two cards are "missing red" -----except that they were bought on Ebay from a seller who said the cards had been framed and hung in a barbershop for about 40 years. They have back damage, and with that much light exposure the lack of color is almost surely fading. Other cards from that group have been graded and labeled as missing color. One of these two shows less gloss where the red should have been, the other doesn't. So one
might actually be missing red. But I wouldn't feel comfortable calling it that even if it passed grading.
The Huggins is an example of an oddity I haven't quite figured out. Obviously the red is there, but with the washed out look something else is either missing or wrong, probably the pink. But the background has some red/pink shading which is a bit unusual. It's probably not fading since the red appears to be just fine. The back has a small stain, so maybe it was mounted somewhere and whatever was used to remove it wiped out the pink. I need to pick up another Huggins to compare the background.
The Beck is a missing color for sure. No back damage or stains. And it's actually missing two colors pink and gray. Most of the missing colors I see that don't have any other issues are missing another color as well as the obvious one.
Steve B