Coming from a Nordic country, the idea of making a snow-/winter-themed army has always appealed to me. However, the more experienced I've become in painting and modeling, the more I have realised how difficult a task it would be to pull off so that I'd be personally happy of how I've done. However, that doesn't have anything to do with your post, so let's get to it : )
First few things, basic things, that I'd like to remind you of:
Thin down your paints, especially when painting white (or yellow, or similar light colours). Always
file/sand mould lines. Especially
when painting white.
Drill the barrels. Simple but makes a huge difference. I personally use a fresh surgical steel blade for that, and not really a drill - works either way.
And then on to the problematics of white and winter:
When painting white, it's pretty much always a better idea to paint off-white and use white only on highlights. If one wants to get professional or adventurous about it, the off-white should be tinted to a freely picked ambient lighting that might be present. For example bluish for daytime sky reflecting off of fresh snow in general.
I'm astounded how pretty much every winter army seems to forget, or rather neglect, the chance to model frozen metal, or frozen armour. Like, for example here
. There might be an idea to explore, now that you've taken on to make a winter-themed army.
It'd be probably a good idea to avoid primary colours, especially red and blue together, when working with large white areas as the dominant colour. At least a blend to secondary colours (orange, purple, and so on) could save it.
Stealth suits are a bit tricky to execute nicely. I like your idea of a ghostly figure in a snow-filled landscape materialising nearby, just a split-second before the air is filled with massive amount of pulse fire. It might be worth it to upsize the bases (if I recall correctly GW has stated that bases can be made bigger, but never smaller. In addition of course to the clause that they should be based on the bases the model is supplied with. But I'd still probably go with 40mm bases, for the modeling opportunities. And out of spite, perhaps.) Imagine how much better it'd look if you'd have room on the base to add some scenery, tank armour fragments still glowing with fusion fires' heat, rubble and stones and stuff like that, covered mainly in snow, but adding an element to reflect to the stealth suits camo-field. It'd give a reason to partially paint the suit with the same colours as the rubble or the armour fragments or even the after glow of the fusion fires.
This ended up being more a random collection of my musings on the subject, and not really a constructive criticism at all. Oh well. I hope you can draw some inspiration from this at least.
Winter-theme is a great opportunity, and please do log your progress here as you get stuff done : )