For translucent colors like your first link I just add dye to nitro to get what I want. The StewMac Finishing book has a pretty good color chart if you want to match color. As always, experiment on scrap.

I've never shot solid colors in nitro (with exception of black rattle can for head plates). I have a friend who is a custom motorcycle painter, he also does guitars using two part automobile finishes. That is a whole 'nother level of finishing that I haven't attempted.

Do you know what type of finish you want? The Reranch cans are nitrocellulose lacquer (often nicknamed "nitro"). Nitro is a traditional guitar finish. Mistakes can just about always be fixed. You must wear a respirator with activated charcoal to absorb the poisonous lacquer thinner. All this and much more is explained in the "Guitar Finishing Step by Step" book sold at the Stewmac website. They are also a source for clear nitro, transparent dyes for mixing traditional guitar finishes where the grain can still be seen through the finish, and a few opaque pigments for mixing with the nitro to make solid colors. I hope this helps. You might want to read the book before making further decisions.
The choice of the chemistry of your medium is the deciding factor as to what pigment systems can be used to mix a custom color. People do guitars with quite a few systems. I think nitro is easiest to learn, but people get great results with other types.