If by plain you mean monochrome, ok then. Otherwise, while being monochrome, the RT panels have excellent fit 'n finish, better than any DIY I have personally seen.
The varying thicknesses have to do with 1) target frequencies, and 2) the material being used. GIK uses mineral wool which not only costs less but requires less "mass", or at least that is my present belief. OTOH, the material is much more difficult to work with for the DIYer.
If you got to RT's site, you will see that there is not only a difference with their different traps, but you can even get different versions of the same size trap to better target HF or not.
Do not paint your panels. There is a specific reason that I used the word "dye" in my first response here, and that is because paint is reflective.