OK, from an artistic perspective, what you generally want is a unifying aesthetic throughout a force. The easiest way to do that is to paint everything the same color. Duh, right?
The next easiest way to do that is to make all the bases match. Use the same flocking, dirt, and base edge color for all units of a given force. That way, even if the paint schemes on the actual minis differ, you've got a uniform element to all the minis. Trouble is, we all have different flock and base fill, and I'd expect it's unreasonable to ask everybody to go buy Woodland Scenics Brown Fine Ballast and Citadel Modeling Grass.
So, there's a third way to do it.
Everyone can paint "mostly" what they want, but everybody ends up with an identical unifying color across each of their individual schemes. The BEST example I can think of for us is "Davion Green". So, for example, let's assume for a moment that we all agree that Davion Green is a 1-to-1 equivalent to GW Snot Green (a bad choice, but roll with it). Here's a list of units that incorporate Davion Green into their paint scheme:
1st Kittery Borderers (green on green camo)
1st Federated Suns Armored Cavalry (Green w/white panels)
1st Capellan Dragoons (green & black countercharged)
20th Avalon Hussars (light gray with tan and green paneling)
42nd Avalon Hussars (light gray with tan and green paneling)
1st Chisholm's Raiders (no cohesive paint scheme...but the unit is nicknamed "The GREEN Giant")
ALL Capellan March Militia units (green with red paneling)
ALL NAIS Cadet Cadre units (flat Davion Green)
ALL Cruicis Lancers units (they all use a base of Davion Green, paneled with other colors)
ALL Syrtis Fusiliers units (flat Green)
...that's a pretty wide selection of units. yeah, it leaves out the Davion Guards, New Ivaarson Chausseurs, Robinson Rangers, Deneb Light Cav, and the Draconis March and Cruicis March Militia regiments...but we're still talking more than half the FedSuns TO&E being available to choose from.
Therefore, my suggestion is to choose a color shared by a lot of units. We agree on a single color from a single supplier (Citadel Snot Green, Vallejo 70.968 #83 Flat Green, etc), and everybody paints some units with that color included. Additionally, we all color all base edges black (no manufacturer equivalency needed for that...). This plan also has the side bonus of ensuring that nobody has to do more than buy a single color of paint - especially if we go with the green since there's a lot of units that just use the flat green with no paneling. That makes a minimal financial effort for everyone.
That's two points of uniformity while still letting people paint the units they want. Make sense?