I start out batch painting, but by the time I am approaching finishing the figures, I have done so much differential work on each that they become individual projects. I mostly do unarmored armies, so better than half the figures are skin and plain wool or linen color. That goes pretty fast. Then you know the advice, "Paint one with a green shirt, another with green pants, a third with green stripes, the next with green fringe..." Yada, Yada. By the time I run through the color palette, stripes, solids, fringes, plaids if needed, different hair colors, individual shield patterns, shading, different weapons, backing up to correct the mistakes (yes, I confess) and so on, I'm not sure that I am actually saving any time.
Concerning the choice of colours. This works for me:
If possible find out the colours of the regions or cities of the army and use them as the basis of the figure colours. For example, for my Venice army I chose Verona, Dolo, Treviso, and Trieste for the Sp and 4Pk and used their colours for the figures (with a few other accents, such as a bit of green). www.iandrea.co.uk/wargames/DBA/venice/index.html. This is easier for the later book armies, as there is more relevant information.
Otherwise you can find some paintings of that era/region and choose colours from them (the painter will have used colours that work well together).
If in doubt, use lighter colours and dustier/earthier ones.