Have your sign open and ready.
Select the brush tool.
Select a Hard Round
brush sized around 14px
Create a new layer above your sign.
Select your desired foreground colour. I'm using a darkish purple.
Draw a few scribbles where you want to pixelate (as shown below).
Go to Filter > Pixelate > Mosaic
Set the "Square" size to around 21 pixels. Then press "OK".
Go to Filter > Sharpen > Sharpen
Sharpen it again one more time. Then lower your pixel layer opacity to around 60 or above (depending on your sign, mess around with it and see what looks best).
This is my result.