The original reported behavior called out that filling individual channels did not apply to LODs when LOD Model Painting = False.
With changes implemented in 4627582, the initial fill will reach LODs; subsequent fills will not.
Original notes from Nori:
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)
LOD0 is filled with Yellow.
LODs 1 and 2 are filled with Red
All LODs are filled with the selected colors after Filling with the selected Color
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-66702 in the post.