A Godot shader to replace one colour in a palette with another (with a mask support for additional colours)

Godot Palette Replacement Shader.md

You must create a palette of colours and set them to the shader's parameters before use in the manner used by the Sprite script source code palette.gd

palette-replace.shader

shader_type canvas_item;

uniform highp vec2 mask_position;
uniform highp vec2 mask_size;
uniform highp vec2 sprite_size;

uniform mat4 global_transform;

uniform sampler2D palette_swap : hint_albedo;
uniform sampler2D palette_with : hint_albedo;
uniform sampler2D palette_swap_mask : hint_white;
uniform sampler2D palette_swap_with : hint_white;

int size(sampler2D tex) {
	return int(textureSize(tex, 0).x);
}

vec4 lookupPaletteSwap(sampler2D swap, sampler2D with, vec4 currcol, vec2 TEXTURE_PIXEL_SIZE) {
	for (int i = 0; i < size(swap); i++) {
		vec2 lookup = vec2(float(i) / float(size(swap)), 0.0);
		vec4 color = texture(swap, lookup);
		vec4 rcolor = texture(with, lookup);
		
		if (currcol == color) {
			return rcolor;
		}
	}
	
	return currcol;
}

void fragment() {
	vec4 currcol = lookupPaletteSwap(palette_swap, palette_with, texture(TEXTURE, UV), TEXTURE_PIXEL_SIZE);
	
	vec2 origin = vec2(global_transform[3][0], global_transform[3][1]) - sprite_size;
	
	vec2 rect1Pos = (origin + mask_position) * SCREEN_PIXEL_SIZE;
	vec2 rect1Size = mask_size * SCREEN_PIXEL_SIZE;

	vec2 normUV = vec2(SCREEN_UV.x, 1.0-SCREEN_UV.y);
	
	if (normUV.x > rect1Pos.x && normUV.x < (rect1Pos + rect1Size).x) {
		if (normUV.y > rect1Pos.y && normUV.y < (rect1Pos + rect1Size).y) {
			currcol = lookupPaletteSwap(palette_swap_mask, palette_swap_with, texture(TEXTURE, UV), TEXTURE_PIXEL_SIZE);
		}
	}
	
	COLOR = currcol;
}

palette.gd

extends Sprite

func generate_image(colors):
	var swap = Image.new();
	swap.create(colors.size(), 1, false, Image.FORMAT_RGBA8);
	
	for i in range(0, colors.size()):
		var color = colors[i];
		swap.lock()
		swap.set_pixel(i, 0, color)
		swap.unlock()
		
	var texture = ImageTexture.new()
	texture.create_from_image(swap, 0);
	
	return texture;
	

# Use your own custom colours here

func generate_palettes():
	var swaps = [
		Color(240.0 / 255.0, 128.0 / 255.0, 0.0, 1.0),
		Color(200.0 / 255.0, 80.0 / 255.0, 0.0, 1.0),
		Color(136.0 / 255.0, 48.0 / 255.0, 0.0, 1.0),
		Color(112.0 / 255.0, 56.0 / 255.0, 0.0, 1.0)
	]
	
	var withs = [
		Color(54.0 / 255.0, 54.0 / 255.0, 54.0 / 255.0, 1.0),
		Color(42.0 / 255.0, 42.0 / 255.0, 42.0 / 255.0, 1.0),
		Color(33.0 / 255.0, 33.0 / 255.0, 33.0 / 255.0, 1.0),
		Color(12.0 / 255.0, 12.0 / 255.0, 12.0 / 255.0, 1.0),
	]
	
	var mask_swaps = [
		Color(0.0, 24.0 / 255.0, 144.0 / 255.0, 1.0)
	]
	
	var mask_withs = [
		Color(42.0 / 255.0, 42.0 / 255.0, 42.0 / 255.0, 1.0)
	]
	
	set_shader("palette_swap", generate_image(swaps))
	set_shader("palette_with", generate_image(withs))
	set_shader("palette_swap_mask", generate_image(mask_swaps))
	set_shader("palette_swap_with", generate_image(mask_withs))
	
func set_shader(param, value, silent = true):
	if (!silent):
		print_debug(param + " " + String(value))
	
	get_material().set_shader_param(param, value)

func init_shader():
	set_shader("sprite_size", get_rect().size, false)
	set_shader("global_transform", get_global_transform())

func _ready():
	generate_palettes()
	init_shader()

func _process(delta):
	set_shader("global_transform", get_global_transform())