My suggestion:
Work out a 'primary' color for the army to tie everything together. Whatever color you want to be the 'main' focus of the force, outside of the green skin of course. If you don't have this, then you're going to run into the incoherent horde situation that happened the first time.
After you determine your primary color, work out some secondary colors, no more than two I would suggest. If you go with red as the primary color, as an example, then black would be one secondary color to use, as well as yellow or blue, whatever tickles your fancy and works well. A third secondary color you can use is of course metallic color (gun metal, bare armor plating, whatever you want to look metal)
Then, bring in tertiary colors such as green (using the red primary color I just mentioned above, green is opposite of it on the color wheel) being used on small parts, and of course a different shade from the green skin shade you use for the orks in this instance, as well as golds or other detail/small points of color. Weathering also falls in to this category with the rust shades or verdigris depending on what kind of metal it is and how corroded you want it to look.
Using this same color palette over the entire force will help tie it all together. You don't have to use EVERY color on each and every model, but the majority to help them all look visually similar and able to be distinguished from another force on the tabletop if they're in a big 'ol scrum like orks love to do while Waaaghing.
I hope that helps. Best of luck sir and I hope to see pictures at some point of your progress.
Take it easy.
-Red__Thirst-