I suppose it is white due to tradition - Saruman was white for so long, he grew accustomed to the title.
Besides, the white is more close to colourless, which is what Saruman, in fact, became, than any particular colour.
Orcs re:
Ahem. That's why, in a sense, difficulties are solved by supposition that Orkish leaders (as opposed to whole mass of 'beast-orcs') are, at least, not depraved of free will, but it does raise another difficulty – it is unmerciful to slaughter them, as they (assuming they have free will) are equals by rank of 'Good Chaps'
And Tolkien obviously tries to solve the problem by a side exit. See:
1. Grishnákh is killed by stray arrow, following his own evil actions (Chance. Maybe even a suicide?)
2. Uglúk is slain in
fair fight, as equal, by [dismounted] Éomer.
3. Shagrat and Gorbag kill each other off
Beast-orcs, requiring direct control of Sauron’s will, disperse by themselves once there is no Sauron around to drive them, and ‘free-willed’ leaders are almost always given a chance – I can’t claim its truth, I can’t back myself up, but I always had a feeling that if only Uglúk prayed for mercy, he had a chance of being spared.
Some meditations on the subject can be found on the main site:
All About Orcs