With Vallejo, more than any other paint,you should follow the manufacturer's instructions for thinning: distilled water. How great is it that they don't repackage some other chemcal and try to sell it to you as THE thinner for their paint! At a couple dollars a gallon, it's the best bargain inthe hobby and you buy it at grocery or CVS type stores. I put some in a cleaned condiment container from take out and cover it when not in use. I have a few eye droppers to transfer it to the paint. For clean up, I use tap water.
You'll want several flesh tones. My base is beige red with their games like dark fleshtone as my primary shading agent. I use sunny skintone, dark flesh and one other shade as additives for the mid tones and pale flesh for highlights.
As mentioned, when you first use them or after they've been sitting a while, they really need to be shaken vigorously, banged against the heel of your hand, shaken some more then really shaken.
Old fashioned washes really don't work with these, you will need to learn to apply the thinned paint in very thin layers to get shadows and highlights.
Check out their website and click on "Model color" for a good tutrial on thier use.
Specific colors for Romans? Can't help you there.