The plastic ones, the good plastic ones at least are removable. The main body of it is permanently fixed with silicone, and the comb itself is removable so you can adjust it's height and remove for cleaning.
Generally, whenever mine got 'dirty', I'd just brush them off with a little bottle brush I'd keep in my hood. The stuff that comes off would just end up in the skimmer or sock if you have one.
For the rear walls, you can paint the back of the tank, on the outside with pretty much any paint, but water based would remove easier in the future.