It would be nice to be able to adjust the color saturation of all the symbols in a layer simultaneously, much as can be done with transparency already. Many users have no doubt used Pro's (and ArcMap's) transparency setting as a kludge to fade out a layer's color saturation, but of course it's not quite the same thing and often has undesired side effects (like making things transparent). Adjusting color saturation is a common cartographic technique to reduce the impact of base map (or non-subject) features. You can, of course, adjust each symbol's saturation individually, but what a pain! There's a nice, big blank spot under the transparency slider (Feature Layer > Appearance > Effects) for a saturation slider!