I don't even know that such things exist lol
a. If it was intentional, then it's likely that it's a condition that cannot exist along side shininess, as it already switches the color palette of the pokemon.
b. If it's a simple texture error, there's a possiblility that it is counted after the shininess factor.
What the last point meant was that the game first decides a pokemon's shininess, and then decids whether it's monochrome. Which means the shiny's color palette, and not the normal version's palette will be swapped, and not go negative.
All theory though.