Pixel-Composer/shaders/sh_palette_shift/sh_palette_shift.fsh

//
// Simple passthrough fragment shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform vec4  palette[64];
uniform float paletteAmount;

uniform float shift;

void main() {
    vec4 c = texture2D( gm_BaseTexture, v_vTexcoord );
	
	float minDist = 999.;
	float index   = 0.;
	
	for(float i = 0.; i < paletteAmount; i++) {
		float _dist = distance(c.rgb, palette[int(i)].rgb);
		if(_dist < minDist) {
			minDist = _dist;
			index   = i;
		}
	}
	
	index = mod(index + shift, paletteAmount);
	gl_FragColor = palette[int(index)];
}