I think Absol's shiny form is pretty unique with these red dark colors. See like that, this red is kinda aggressive, reflecting itshigh amount of strength.
Spiritomb's shiny form concept remind me the shining colors of the universe. Because of its types and its colors, that remind me the nebulas floating in the universe!
I must admit that a shiny Spiritomb looks like it has chosen a good path rather than a dark one, it seems to have been corrupted by the light and finds itself in a class where it isn't a real dark pokemon anymore but it isn't truly a ghost or any other type of pokemon, Spiritomb are a very interesting pokemon as they tend only to show up in dark places so having a shiny version which is so bright would make it an outcast of some sort? the other Spiritomb not liking it because it has this corrupted light look to it but I do find it interesting that it resembles a nebular.
A shiny Absol is something someone would have to want to see in there times playing the pokemon games, while I agree that the red makes it look more aggressive and dominant you have to remember that and Absol is a soldier of the night, rather than being a thuggish pokemon and picking fights wherever it can find them, an Absol is pokemon of peace and helps humans when a natural disaster is about to take place it tells the people of the impending doom and guides them to safety! Of course an Absol being mistreated is something to gasp at doing show would make people understand how much power one can have and never to mistreat pokemon that even though look shy and observant can still be a vicious battler.