Godot 4-color shader

4-color.shader

shader_type canvas_item;
render_mode blend_mix;

uniform vec4 color_1 = vec4(0.784313725, 0.788235294, 0.262745098, 1.0);
uniform vec4 color_2 = vec4(0.490196078, 0.521568627, 0.152941176, 1.0);
uniform vec4 color_3 = vec4(0.000000000, 0.415686275, 0.000000000, 1.0);
uniform vec4 color_4 = vec4(0.015686275, 0.243137255, 0.000000000, 1.0);

uniform float offset = 0.5;

float to_grayscale_average(vec4 pixel)
{
	return (pixel.r + pixel.g + pixel.b) / 3.0;
}

float to_grayscale_luminosity(vec4 pixel)
{
  return 0.21 * pixel.r + 0.72 * pixel.g + 0.07 * pixel.b;
}

vec4 colorize(float grayscale)
{
	if (grayscale > offset * 1.5) {
		return color_1;
	}

	if (grayscale > offset) {
		return color_2;
	}

	if (grayscale > offset * 0.5) {
		return color_3;
	}

  return color_4;	
}

void fragment() {
  vec4 pixel_color = texture(SCREEN_TEXTURE, SCREEN_UV);
  COLOR = colorize(to_grayscale_luminosity(pixel_color));
}