I use a 3/16 poly or nylon rope for my cores. that gives me a 9/16 ish 12 strand mecate, which is what I like. If you want thicker use 1/4 for core, that will give about a 5/8 finished diameter. You can also use 1 or two strands of para cord as the core for the same results. I only go 8 strand if the customer has small hands and wants a 1/2" mecate. If you use to big of core in the 8 strand it feels to mushy to me and you can see the core.
As far as colors go, what you said would look good to me, but colors are like a room full of girls, which one is the prettiest?