Retro handheld style post processing shader

Post.frag

//
//  Pass-through post-processing shader template
//  Verto Studio 3D
//  Created by Mike Farrell
//
//  Please read the shader section of the user-guide
//  for more information on shader programming
//  within Verto Studio

precision mediump float;

in mediump vec2 textureCoordinates;
out vec4 fragColor;

uniform sampler2D passInputTexture0;

const float pixelSize = 4.0;

void main()
{
    vec4 color;
    color = texture(passInputTexture0, textureCoordinates);

    //uncomment for an example of a simple sepia-tone 
    //post processing effect
    float grey = dot(color.rgb, vec3(0.299, 0.587,0.114));
    
    vec4 colors = vec4(
      0.2,
      0.4,
      0.6,
      0.8
    );
    
    if(grey < 0.2)
      grey = colors[0];
    else if(grey < 0.4)
      grey = colors[1];
    else if(grey < 0.7)
      grey = colors[2];
    else
      grey = colors[3];
      
    //pixel grid lines
    if(mod(gl_FragCoord.x, pixelSize) < 1.0)
      grey = 1.0;
    if(mod(gl_FragCoord.y, pixelSize) < 1.0)
      grey = 1.0;
      
    color = vec4(grey * vec3(1.2, 1.0, 0.8), 1.0);

    fragColor = color;
}