Shared (every palette-coloured scene — and the backdrop)

Param	Default	What it does
hue	0.0	Rotates the gradient sample coordinate (the pre-existing hue knob). Scene only.
saturation	1.0	Scales chroma toward luma. 1 unchanged, 0 grayscale, >1 oversaturated.
palette_mix	0.0	A/B crossfade position (see below); 0 = palette A.
palette_steps	0.0 (off)	Quantizes the gradient into that many hard bands — see Hard bands.
palette_contour	0.0 (off)	Darkens a hairline at each band edge where the ink actually changes. Fragment-field, reaction-diffusion, shape-field and warp-mesh only; inert elsewhere — same section.

saturation and palette_mix are one binding with two consumers: the active scene and the background pre-pass. You write them once and the whole frame answers — see the backdrop. hue is the exception, and deliberately: it offsets the scene’s sample coordinate, and the backdrop has its own coordinate in bg_hue.