My 2 cents. I think as said above, that Sandtrooper are regular Stormtroopers with a little modified armor to be ready for more hostile climate and more dangerous planets. They have for it heavy weapons, backpack to survive long time outside of base.
They are placed on distant planets with limited supply routes, with longer patrols they don´t have time to wash armor or get new from supply. They replace it only when armor is really damaged. May be these soldiers are more skilled to survive in hostile environment, something like basic commando training. I believe, that on imperial planetes or ISD they have supply with replacement armor and for sure some place, where you can put armor to wash it chemically not by hand like todays soldiers need clean own boots
Why sandtroopers these black lines on greay areas on helmets? To have less holes in helmet and prevent from dust get inside of bucket.
As for Endor I think, when Emperor said, he sent here the best troops, he didn´t lie. He was crazy but not stupid. But probably in chain of command somebody made mistake or it was rebel scum, who changed few details about designation. Instead of sending here elite troops, he sent rookies. I would expect on Endor some elite jungletroopers with camo on armor Not white stormies, that can be harmed with small stones and little bears