...in dark brown tones.? What color combinations do I use to get a dark brown mediterranean rustic look?

Use exterior wood staings, lights, darks..apply with sponge to get look you want. Mottle here, there, wipe here, there..practice makes perfect.

first if they are clay pots give them a couple of coats of PVA.... no.1 it will seal your pots inside and out and prevent them from cracking in winter if you use acrylic paints either from art shop or hardware shop that will stay on in any weather...for a nice dark rich brown mix red and black till you get the right shade and even add some orange to it..depending on what colour you want