I think Gallente might be green versus sandy brown (if the Navy Issue ships are anything to go by). In the battle of Caldari Prime in 110, I always pictured the Fed soldiers being dressed in white, to contrast with the Caldari's black (I think that's the colour they wore in Cities of Refuge).
The Federation military is a different beast to the Caldari Navy. Being larger, and directly representative of the Federation's government (ie. answers to the President), it's an influence projection tool, whereas the State would use the eight security forces for that. We know that Caldari schools are pure practically-minded, and I explored elsewhere that Federation officers have to be well-rounded, politically and socially minded individuals (what I'm finding out as a student officer for the Royal Navy at least) which could contrast with the Caldari being more outright, purely militaristic.
The Gallentean military also takes part in humanitarian endeavours as well, civil affairs and whatnot. I get the sense the Caldari Navy/Army is purely focused on combat.
That is one thing I'm slowly learning actually. An RN Lt warned me at HMNB Clyde, as we were going to get passes to get to the secure area of the base from the MOD office...
"Okay, these guys are civilians, so they have no sense of humour"
Applying this to EVE, perhaps Gallentean military officers are more relaxed and friendly, with this whole emphasis on "well-rounded individuals", but being able to get into a professional mindset when necessary. This fits in with their luxurious, hedonistic societies. Caldari would be your more "Sir yes sir" bellowing.
I had a drink with the officers of the HMS Vanguard there, in fact. It was laughing and joking, extremely relaxed, and even treatment of the captain (an RN Commander) was the same, except they called him "sir". RN officers call each other by their first names except in the presence of ratings. Even senior ratings, like Chief Petty Officers, tend to be called by their first names by superiors, usually nicknames (Scotty for Scott, Mitch for their surname being Mitchell). The RN is overall much more relaxed than the Nelsonian image people have, especially compared to the draconian British Army and Royal Marines.
Then again, the FNA corp desc was updated on the wiki (and in char gen) to state that "it was draconian even for a military school". So who knows.
EDIT -
http://www.youtube.com/watch?v=DyWQinmBjBk&feature=related A recruitment video for the British Army.