Thank you guys, I got it. My shader is a black and white noise (like the old television). And this is what I did
float saturation = 0.5;
float contrast = 1.2;
if (discolor)
{
// Saturation
vec3 greyscale = vec3(text.r + text.g + text.b) / 3.;
text.rgb = mix(text.rgb, greyscale, saturation);
// Contrast
float midpoint = pow(0.5, 2.2);
text.rgb = (text.rgb - vec3(midpoint)) * contrast + midpoint;
}
// Reduce the opacity of the color by modifying the alpha component
text.a *= 0.5; // Adjust the value
COLOR = text;