for seam filling I gue 2 methods at once, the over glue, so ti spills out of the seam and you can slice it off, then I use testors white putty.
One of these days im gona put out a little tutorial on filling seams for gundam kits.
As far as the colorings... Satin = bad, actuially.
If you ever watch the series, they seem to be all shiny and such, but the furthur you get along, the more you start to see that its quite the opposite. The best example being the first ric dias in zeta gundam, the black one
Just like anything else military they tried their best to use camo on vehichles. The same goes for gundams, they would NOT have a shiny satin finish, if anything it would be a flat paint to cover the metalicness of the frame of the mech, which is why you paint something like that. Desert units were more painted tans, jungle units greens and browns, artic units white. Also, space units, though alot of the times in mass produced colors, whenever possible would be done in camo to fit the scenery as well.
Ultimatly, you can give it a 'show room finish' like you would a car model, but that wouldnt be the norm for any mech that was combat bound.