At first, I wasn't even aware that there was such a thing as a Mountain Giant in D&D, but a quick check on Google shows that they first appeared in the Fiend Folio - whilst the D&D Wiki provides a useful summary:
Mountain giants are violent as a result of their history. Banished by their hill giant ancestors, they were forced to drive out the stone giants from their northern ranges. They regularly mock-fight amongst each other. However, they prefer not to start a real fight without a genuine reason (although they will jump into a fight that's already started without hesitation). As such, they may not act anymore violent than an average human on a good day.
So basically, we're looking at a hill giant variant, similar to George - in fact the Fiend Folio illustration looks startlingly like George - but IMHO it looks much more like a Frost Giant, complete with furs, battle axe and Nordic helmet, and that dictated the paint scheme. The paint job was fairly challenging - just a simple base coat of white, then a wash of GW Draaken Nightshade, then lots and lots of drybrushing with various shades of light blue and then white. I wanted to give the impression of the leather jerkin and helmet being cold, so I mixed in some light blue into the usual leather colour, and I think that gave the right effect. The shaft of the axe was problematical - I originally painted it a light yellow brown to suggest pine, but it looked almost too bright, so again I mixed in light blue to a light brown and then added white to try and get a frosted effect.
I don't think its a great sculpt - its a bit static, and there is a definite coarseness about the figure, though nothing as bad as early Asgard figures. Still, I'm fairly pleased with this outcome - it will definitely work as a default Frost Giant figure for encounters.