`z = acf(x, y) / (3 sigma); color_rgb = z < 0 ? z * vec3(4, 2, 1) : z * vec3(1, 2, 4);` That's really it. The (4, 2, 1) is the oversaturated orange color, i.e.

`z = acf(x, y) / (3 sigma); color_rgb = z < 0 ? z * vec3(4, 2, 1) : z * vec3(1, 2, 4);` That's really it. The (4, 2, 1) is the oversaturated orange color, i.e. it would progress as (1, 0.5, 0.25) -> (1, 1, 0.5) -> (1, 1, 1). None of the tricky HSL/HSV schemes worked better than this.