I think mixing multiple primary colors and the dominant silver is a bad idea... the rainbow look is not a pretty one. My favorite Ork artwork is often the older 2nd edition / Epic stuff, where they are all very looted looking but very simple in color scheme ie. Yellow with black flames, or red, or whatever.
If you want some armor plates to look different I suggest more neutral colors, ie. Catachan Green, Graveyard Earth, black/grey, and maybe even just doing different metallic techniques like antiqued brass or something.