If there's a 15-color limit, then it would be worthwhile to go through each finished Smogon BW sprite that has already been made (especially the shinies and shiny megas, as I have recently noticed), and tweak as needed.Unfortunately, this will break the 15-color limit and I think it might be that reason was done as this at first place.
For example: Shiny Mega Metagross, Shiny Mega Gyarados, Shiny Mega Salamence, and Shiny Primal Kyogre. (All of these have some colors that are almost identical to other colors in use, but when performing a 'fill all of the same color' action, they return as different colors, and are thus exceeding the color limit.) Some sprites, such as Shiny Mega Blaziken, have elements that do not appear in the standard sprite (in the case of Shiny Mega Blaziken the fire ribbons are inconsistent, and in the case of Shiny Primal Kyogre, there are colors leftover and textures not transferred from the standard sprite). In many cases, there are also colors left over from the standard Mega's sprite, adding additional colors.