Assuming that you are using the 'TheGridSky' skyscreen, what you wish to achieve simply is not possible. It is not possible to use a PostEffect on a single part of the designated area, and the TheGridSky can't have a same color setting unfortunately. But if you want a pixelated effect use this code:

pixelateEffect = BuildMaterial{
shader="PostLed",
shadersettings={_Scale=5}
}

AddPostEffect{
depth="background",
material = pixelateEffect
}