In gen 7, it looked like this
But for Gen 8, it looks like this:
Now I think I see the difference with the shading, but I want to be 100% sure before I go posting wrong info into the Error Page on Meta. This is all according to the DB's Cosmog sprites.
Sorry if this is Trivial or a low quality question, but I want to make sure I don't post wrong info.
The difference between the normal sprite and the shiny sprite is the same in Sword/Shield as it is in S/M and US/UM. It is, however, different in Pokemon HOME. You can see the differences below, with the normal sprite on the left and the shiny on the right.
The Sun/Moon and Ultra Sun/Ultra Moon sprites: (images from the DB)
The Sword/Shield sprites: (images from this website)
The HOME sprites: (images from this website)
For the S/M, US/UM, and Sw/Sh sprites, the only difference seems to be that the color of Cosmog's cheeks change from blue to pink/red. For some reason, instead of having this difference, shiny Cosmog is simply lighter than normal Cosmog. I'm unsure as to why this is.