I prefer to paint my actual underneath color, whatever it is, and then flat coat the whole project.
I thin down actrylic paint such as tamiya gloss or gunze gloss white, perhaps touch a drop of grey to it or even tan to show a dirtier white. spray it on almost covering the whole thing, but leave a hint of the base showing here and ther where it would come into contact with the ground and crews, wearing the whitewash efffect away.
Let this dry for an hour or so and use common alcohol and a wide brush to wipe some of this off, usually in a downward motion. The effect is striking, but dont overdo it. On Hoth the dominant color (actually about the only color) was white so keep the alcohol removal to a minimum.
Apply a flat coat when your satisfied, and then go over the whole model with highly diluted (30:1) Prussian Blue artist oil to shade it, toning down the white and making it look used. on the bottom side you can apply a typical umber wash for dirt, and maiintenence induced grunge.
HTH
David
Build to please yourself, and don't worry about what others think!
TI 4019 Jolly Roger Squadron, 501st Legion