I've been doing a bit of research on this recently. It's driving me crazy.
There is far more the their history than this but, for these periods, here's what I've been learning.
In the mid-late 1800s they wore the blue long coat, white pants, leather belt and "webbing" (wasn't really webbing) was black leather.
http://www.beau-geste.com/imagesnew/beau-geste2.jpgThey tended to also wear two over-the-shoulder bags, one with a 2 liter canteen. This was usually black but sometimes khaki. Other side had a bag and I don't know what they put in there.
Kepis were tan but bleached to white in the sun or had white covers in gloomier regions. The French liked tall hats with covers on them - even in the Sharpe's Rifles series you can see this (Napoleonic wars). Eventually, they became just white kepis, the Legion's trademark (Sgts. and officers wear black with red tops and wore red pants around 1900).
They wore this uniform in Camaron, Mexico (the Legion's most celebrated fight). Good example:
http://listverse.files.wordpress.com/2009/08/danjou1-tm.jpg Sometimes they also wore sombrero hats, but that's probably not part of your VSF scheme.
In specific areas sometimes they had different local uniforms - there are photos of legionaires in Africa wearing all light colors.
By WWI they wore standard French uniforms because that was what was available. This just meant everyone got red pants after a while.
Post-WWI the jacket shortened (like the Foundry and Artizan minis) and I've seen blue and I've seen sand colored.
I have a bunch of figures myself and I'm trying to figure out which color to use. The blue looks nice but that was iconically used for the longer, pre-WWI jackets.
By mid- to late-WWII they wore American uniforms with FFL patches simply because the US was providing their equipment at the time. (I think one of the reason that the French have such an undeserved cowardly reputation is because their soldiers were incorporated into the UK and US armies so there are very very few images of identifiably French soldiers from WWII).
Post-WWII they wore khaki, adopted green berets somewhere along the way, and now they wear a surprisingly bland 3-color camo and look like most modern soldiers (
http://img300.imageshack.us/img300/6300/610xclo.jpg). Kinda boring - I would have expected something more artistic, something with more flair from the French. Go figure.
Good movies to check out - Beau Geste (old flick, I think it's in black and white) and a surprisingly good Gene Hackman movie set just after WWI (in color), and Indochine (in French, about Dienbienphu where they finally got what was coming to them).
For VSF I'd go with the long blue coats, white pants and kepis for the troops, red pants and kepis for the NCOs and officers, and if you can highlight your blue coats up enough to get good contrast with the black webbing I'd use that because it is more authentic. Khaki webbing would be alright, too, and would provide a nice contrast to those richly colored blue jackets.
I would not use brown becuase it introduces another color to the figure and would detract from the contrast, but that's just me.
This looks right in terms of colors:
http://www.unfeasibly.co.uk/page8.htmAnyway, I hope this helps. I'm working on the same kind of thing but straight historical, not sci-fi, so I have less artistic license (you lucky guy).