Ha, well I think the Napoleonic era has some really detailed and colorful uniforms to paint, so I've got quite a few figures from that end of the 18th and beginning of the 19th century period. Even if you want to be historically accurate that era still gives you a lot of freedom in your color choices. Take the French Hussar, for example. You've got 14 different regiments each with their own color scheme to pick from. The dragoon isn't quite as flexible, still I could have gone with scarlet, crimson, pink, yellow or orange for the details. Plus the French Revolution and rise/fall of Napoleon provides some interesting historical background. So that and the ancients (Romans/Greeks) are probably the two historical periods I find most compelling. I've also got a random assortment of knights, pirates, and samurai somewhere in my pile of unpainted figures.
My dad's area of interest is the Civil War. But back when he painted he said he got bored of painting dark blue or grey over and over again. He started to paint mostly zouaves because at least that gave him some color variation.