First and foremost, regarding Sunlight, congratulations! It’s great to hear that she’s doing so well. May the hatchling be happy and healthy!
As for your question, it’s actually entirely possible. Although shininess and standard coloration are both defined as specific palettes according to breeders, the possible colors a pokémon can be is actually more of a spectrum. Pokémon can possess pretty much any combination of colors: limbs or appendages that are actually of the shiny variant, pelts that are shades lighter or darker than they should be, and even scales that display a cross of both. It’s just rarer for it to happen as such, but for a pokémon to be born like that? It’s certainly possible.
However, for evolutions, you’ll need to be a bit more careful, especially if the preevolution was not colored that way. In some cases, yes, the process of evolution could trigger a unique sequence of genes that results in odd coloration compared to the preevolved form, but in some cases, this could be an indication that there’s an underlying health issue. It wouldn’t hurt to take your haxorus out for a thorough check-up, just to be on the safe side.