I mean, it’s not just an illusion. The 4 - 8 - 16 colours show that according to the formula the colours change squarely from the centre. So the color in the corners is always brighter if you think in a circle from the center.
That’s why I have the circular formula.
// function definition (circles)
float circ(float d, float x, float y){
return d + sqrt( x * x + y * y );
So you can create almost any form of color change if you have the right formula.
In my addons I use a wide variety of functions to create 3D shapes. Maybe you can get something out of it.
Addon. Produces almost infinite many time-varying geometries with functions