Saturation

shader_type canvas_item;

uniform float saturation : hint_range(0.0, 1.0, 0.1);

void fragment() {
    vec4 tex_color = texture(TEXTURE, UV);

    COLOR.rgb = mix(vec3(dot(tex_color.rgb, vec3(0.299, 0.587, 0.114))), tex_color.rgb, saturation);
    COLOR.a = tex_color.a;
}