Filling vertex color with a mask doesn't fill fillcolor but maskcolor to its LOD models incorrectly
It seems that this is caused just by a typo in MeshPaintHelpers::SetInstanceColorDataForLOD().
I guess InitFromSingleColor(MaskColor...) is wrong and it should be InitFromSingleColor(FillColor...).
JFYI, As a test, I shelved a super small CL to fix it on 4.21. (4575538)
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-66702 in the post.