Fragment field, reaction-diffusion & shape field — where the field sits in the gradient

Param	Default	What it does
color_span	0.6 (fragment and shape field) / 0.85 (RD)	Multiplies the scene’s field level to set how much of the gradient it spans. Low = a cohesive single-family mood; high = a wide sweep.
color_center	0.0	Where that window sits in the gradient. Slide it (e.g. 0.1 + treb*0.2) to move the tonal centre. Cyclic — see below.

A warm, cohesive field: a warm palette + a low color_span.

color_center is a cyclic coordinate, and it wraps rather than clamps. It rides the same repeating gradient described above, so pushing it negative to reach a palette’s dark end lands the field in that palette’s bright stops instead and the picture gets brighter — the opposite of the intent. This is not hypothetical: it cost the content lane three rendered iterations of chasing exposure, contour density and the palette ramp, all downstream of a cause that was none of them (design-backlog 0027). Keep the centre inside 0..1 and know that -0.1 and 0.9 are the same place. To actually darken a field, change the palette’s stops or the scene’s own level param — glow on the fragment field and reaction-diffusion, brightness on the three particle scenes. Not exposure: that is the whole-frame stop, it moves the backdrop and every other stage with the field, and bloom_threshold is measured against it (ADR-0080).

Reaction-diffusion needs a much larger color_span than the fragment field. color_span multiplies each scene’s native field level, and those ranges differ: the fragment field runs the full 0..1, but reaction-diffusion’s field level (the Gray-Scott V concentration it colours by) only reaches roughly 0..0.4. So at the RD default 0.85 the sampled coordinate never climbs past about 0.34 — the warm end of a custom gradient (any stop with at above ~0.4) is never reached, and a teal -> orange -> gold reef renders all-teal. To span a full custom gradient on RD, set color_span to roughly 2.0–2.5 (the shipped coral presets use ~2.2); the exact value shifts a little by regime, since a high-fill pattern drives the field level higher than a thin maze. On the fragment field, keep color_span in the usual 0.2–0.7 range.