So, I decided to take the K. Rool costume and play matches against the dark green palette swap for Bowser, as well as Dedede, to compare textures on the scaly skin and gold, respectively.
My findings were that the color of the dark green skin on the costume matches that of dark green Bowser almost perfectly, but there's a distinct 'shiny' feel to the costume's skin...it has the appearance of being slick and slimy, whereas Bowser's appears drier and more coarse. With Dedede, his gold is much brighter than K. Rool's, which almost has a bronze appearance in comparison.
Before release, I remember thinking that those textures would be re-used for a playable K. Rool model, but I guess that didn't end up happening. The costume has textures all its own, for worse or for better.