Sun 30 May 2021, 23:54
According to standard NATO iconography, they mean either "degraded/damaged" or "destroyed."
Of course, there seems to be little relationship in the remaining strength of the in-game units and whether they are yellow or red. Most are red -- which implies incapable of action.
I don't know for sure, but it seems likely to me that these codings would at some point have been recalibrated to the reality of units in the field, post-1997. Everyone at NATO HQ and elsewhere knows there aren't going to be any more 20,000 man divisions!
Author, Central Poland Sourcebook -- now available on DriveThruRPG