Source - https://godotengine.org/asset-library/asset/122

bcs.shader

shader_type canvas_item;

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;
}