Check out these painting guides.
http://www.minairons.eu/en/search?search_query=Painting+Guide+&orderby=position&orderway=desc
There's now a fourth guide [Left-Wing Militia] to add to the previous three of:
- Republican Army
- Nationalist Army
- Moroccan regulars
I had previously tried to "order" (they're free, but you need to go through the process anyway) these guides but never managed to get to the downloads. It seems that I was not alone: there is a thread today on TMP dealing with just this: http://theminiaturespage.com/news/?id=1621261420
Basically, if you go back into your order history, you can access these guides and download them directly. Somewhat convoluted, but worth a little effort.
The guides [which are easily detailed enough for most people in 15mm] provide Vallejo paint colour numbers, including alternatives for those looking for a little variety within their units.