Theme's (a search for new ones)

I'm not sure I understand what you're asking for...Do you want ideas to differentiate each faction in your game (10 is alot and would be epic battling) or do you need to differentiate the units within each faction's army (cavalry, footment, etc)? If it is the latter...

I'm not sure if using different colors for variable units within the same faction is a good idea. I imagine tracking your units against your opponents may get difficult if each faction's unit used the same color scheme to mark unit function.
What about using different bases for the units each faction commands (like in Rune Wars - it's been awhile since I played another with variable units...). Some units have squares, others have octagons, each used to reference the particular unit's order in battle etc.
Another idea is some kind of pip or banner on the unit (base) that would denote their function.
If you're down to get a little abstract, what about using shapes to mark the different units (snipers are triangles and tanks are hexagons...)? Each faction has it's own color and each unit it's own shape.

Good luck!

I'll try and toss some ideas out there..

-Robot army.
Grey might be a fitting color, could be quite efficient but lacking in creativity. Could overall be pretty strong in offense and defense, but with unique openings for enemies to exploit because they are robots.

-Commando's.
Green might be most suitable for them.
They could focus more on stealth and guerilla warfare, but have some strong explosives and suicidal rambo-ish front line warriors. Only light vehicles though.

-The fleet.
Blue seems a natural match here.
Using airships, submarines, hovercrafts, marines, artillery support from battleships, etc. They would have a diverse bag of tricks but could be weaker inland and strong at shorelines.

-Pyromaniacs.
Orange could be their color.
Their forces could be mainly basic army stuff, but in addition to that they get flamethrowers, flame tanks, napalm bombs and such. They love fire.

-Airforce.
White might match here.
They could have acces to basic stuff, but their focus would be air. So they can easily drop off supplies, infantry and such and provide air strikes.

-Aliens.
Purple might be fitting.
They could have more powerfull troops but would field much less troops.

-Defensive.
Yellow might work I suppose, They are always on yellow alert?
Trenches, bunkers, slow heavy tanks, rockets.. they exell in defense but are kind of a lumbering giant which makes conquest harder. They are better at keeping ground than gaining it.

-Steampunk.
Brown goes with them.
Using steamtanks, primitive robots, airships, heavy artillery, trains, steamboats, fast moving cavalry with cowboys that wield repeating pistols, snipers, and other stuff you might come up with.

-Agressor.
Red of course.
They could have tanks, heavy missles, fast infantry with heavy rifles, grenade launcers, etc. They would deal great damage but might risk their own troops in the process, being a bit suicidal in their thirst for conquest. Kind of a modern horde of raging barbarians.

-Evil corporation.
Black suits them.
They could field more police like troops. SWAT teams, snipers, attack dogs that can reveal stealthed units, media broadcasts allowing for great radio range, light tanks, attack helicopters that can drop off SWAT troops, armored cars, etc.

I hope this helped although it was a bit of a mix between theme and play style. Maybe it will give some ideas for the possibilities.