Well, it all depends on the race in question...
For the EA we have an "official" paint sceme, well, two actually. "Light grey with blue tim" (Hyperion) and "dark gray" (Omega etc.). And we have indication that the coloured trim does not have to be blue (B1 was grey & red, B4 grey & green, B5 grey & dark blue).
So I'd say the basic grey is fixed, the blue trim can be color-changed (probably depending on which sector fleet or battlegroup or whatever the ships belong to...)
Centauri... we know from the fluff that the "official" purple-gold-grey colors are the colors of the Royal Navy. Individual houses might have different color scemes (Besides my "Centauri Royal Navy" ships I have a small house fleet under construction which I intend to paint red and silver for example)
Narn... two color scemes were seen on the show, the common "red-black-white" and the seemingly oncommon "grey & blue". I always assumes the reddish was navy and the grey-blue ground forces (meaning only used for troop transports). Since the narn have no special fractions like the cenaturi houses, I would expect little variation...
Minbari... well in the show we saw they have blue ships... but even here there is variation. From "light blue with dark blue trim" over "light blue with purple trim" to "dark blue with greenish-blue trim"... so any blusih combination would be possible. The Minbari Protectorate might even have a completely different color sceme for their vessels... maybe green based...
Abbai... well, actually AoG CGI had them dark grey and deep blue, with light blue added on the Lakara only. Mongoose changed this into the blue & orange we see today. One could assume that blue is the Abbai base color, and depending on fleet other colors are used...
Brakiri... we know of the green and yellow... but that may be in use only by the Ak-Habil comglomerate! Other corporations might have other paint scemes... I would still keep to the greenish theme, but "dark green and red" is as applicable as "yellow-green and sky blue"
Drazi... well, all the ships we ever saw were kinda gunmetal-grey with red markings. But since we didn't get that good a look at them one can expect different coloured markings and individual decorations (it would be "drazi" for a captain to paint his ship anything the other captains can't stop him to do... but most will make sure their paint sceme doesn't give cause for too many brawls) Hell, at certain times ships might repait all their marking to either green or purple...
Vree... grey with colors - blue and yellow were seen in the show, but every color should be possible. Just don't paint them like big yellow smiley faces...
Raiders & Civilians... they can paint their ships whatever they want. Civilians will have company colors and logos, while raiders might short garish paint scemes matching the bands name (so the "Bloodstar Pirates" might have dark grey ships with painted on bloow drops and their "skull in a red star" insignia while the "Hells Raiders" might have coal-black hulls with flames around gunports and engines, and the sneaky "Rock Hounds" might have a brown camo pattern to better hide in asteroids... you get the idea)
Vorlons... well, in the show we have yellow with greenish-brown markings and orange with reddish-brown markings. Back in B5W we also had some more skin colors mentioned... Azure and Amethyst...
Drakh... no variation known.
Shadows... no variation at all.
