Gameboy shader made in shadertoy

gameboy.glsl

// Simple GameBoy style shader in ShaderToy
// This require a texture/Webcam input on Channel0 in ShaderToy
// SheatNoisette (2020) - GNU GPL 3.0 Licensed

vec3 gameboy_palette[4];

vec3 gameboyify(vec3 color) {
    // Create palette
    gameboy_palette[0] = vec3(51, 44, 80);
    gameboy_palette[1] = vec3(70, 135, 143);
    gameboy_palette[2] = vec3(148, 227, 68);
    gameboy_palette[3] = vec3(226, 243, 228);
    
    // Gameboy shader
    float diff = 256.;
    float current_diff = 0.;
    int min_index = 0;
    int index;
    
    for (index = 0; index < 4; index++) {
    	current_diff = sqrt(
            pow((color.x) - gameboy_palette[index].x, 2.) +
            pow((color.y) - gameboy_palette[index].y, 2.) +
            pow((color.z) - gameboy_palette[index].z, 2.)
            );

        if (current_diff < diff) {
        	diff = current_diff;
            min_index = index;
        }
    }
    
    return gameboy_palette[min_index];

}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    // Output to screen
    for (int posY = -1; posY <= 1; posY++) {
    	for (int posX = -1; posX <= 1; posX++) {
            float x = (fragCoord.x + float(posX))/iResolution.x;
            float y =  (fragCoord.y + float(posY))/iResolution.y;

            fragColor.xyz = gameboyify(texture( iChannel0, vec2(x, y)).xyz * 255.) / 255.;
        }
    }
}