godot-demo-projects/2d/screen_space_shaders/shaders/BCS.gdshader

shader_type canvas_item;

uniform sampler2D screen_texture : hint_screen_texture, filter_linear_mipmap;
uniform float brightness = 0.8;
uniform float contrast = 1.5;
uniform float saturation = 1.8;

void fragment() {
	vec3 c = textureLod(screen_texture, SCREEN_UV, 0.0).rgb;

	c.rgb = mix(vec3(0.0), c.rgb, brightness);
	c.rgb = mix(vec3(0.5), c.rgb, contrast);
	c.rgb = mix(vec3(dot(vec3(1.0), c.rgb) * 0.33333), c.rgb, saturation);

	COLOR.rgb = c;
}