Figured out a new technique for enriching color-sensitive textures by adding CIE noise via The GIMP (which works with the SRB2 Palette).
The result is richer, more realistic color and less grey for the rocks in Wondrous Windsock as well as introducing a subtle off-white coloring as originally desired for the marble in Lawn World.
This has not been applied to all textures, but only to those where better color performance is critical to the look.