way of limiting debanding shader

Is it possible to limit the deband shader to filter to only specified colors , say if a game just in general has bad skys that is most noticeable instead of getting it to look for everything would it run faster if just looking for colors in a specific range and brightness ?

well, skies are blue, you can add some if conditions and check if color.b is above some threshold, before applying, so effect would apply only to pixels with enough of blue color, mostly skybox and water, but I suggest you to add extra condition to check if all of rgb colors almost close to 0 to deband shadows.