Here are fast concepts of color schemes I would paint on my Mantic Undead Army.
Skeletons would have silver armor (non metallic) and black clothes. There would be rust here and there on metal parts. Definitely shields should be red. As red would be main color for this army (I was hesitating between red, turquoise and violet, red won, so this would be main color for this army). To match color scheme on ghouls and zombies (they to not use any shields), I’ll paint hands and wounds in the same red color as shields. Maybe some standards for zombies would have signs of an army and possibly ghouls would have tattoos on their bodies in that shape. Catapults would have dark brown wood. Here’s color scheme for skeletons and revenants.
Here’s color sheme for zombies and ghouls.
All heroes in this army would have a different color scheme. Instead of red they would have turquoise as a main color to make them stand out. So here’s a necromancer scheme.