They could, but honestly the paint looks pretty good now. When they refurbished the castle in early 2015 to get ready for the Diamond overlay, they painted it then, and removed the pink coloring. (Can you believe that was four years ago now?!)
I was referring to the pre 2005 paint job, that looked like this (taken 1994):
It used to be a grey/pink color scheme, with a lot more grey. Now it's a pink/gold, with an excessive amount of pink.
Though I imagine if they went back to the proper colors, many fans who weren't around for them would think Disney "ruined" the castle.