Sorry you haven't had any responses yet Damnosus. Your painting is extremely well done, you have really nailed the eyes. I can't do eyes properly to save my life!
OK, first the base. If you do anything light-coloured, then I would go with snow. Keep it simple and contrats nicely with the models, without taking anything away from them attention-wise. You could do the bottom edge of the snow's rim with a very, very light electric blue, but that's all that I would suggest.
Next, the warrior. I think you have some great colours that work nicely together and you have them in the right places. Personally, I would consider doing the sash in orange, sort of as an army identifier that would also really make them pop on the tabletop.
And the gun. If you are ticking with a very simple, three-colour scheme, then leave the gun primarily black (or a very dark silver) with faint Boltgun/Leadbelcher highlighting. You could do the two chambers differently, maybe washed Boltgun, but again that is all I would do.
Gems on DE I always do in dark purple with a gloss finish. You could do them orange, if you weren't doing anything else the same colour, but dark purple would match up nicely with the hair.
Whatever you do, make sure we see the finished product!
Cheers.
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