If you're looking to match a color, and have a sample -- I also suggest Sico they make a product called Corrostop which is tintable to anything in their regular color chart, and I suspect that they also have the color matching capabilities as others. Basically the same as Rustoleum - if not an improved formula. Direct to metal, rust inhibiting, alkyd.

For the body of my car, I'm using Sherwin Williams DTM. Again, industrial/marine, direct to metal, rust inhibiting alkyd -- tintable to anything in their chart -- or custom/color match.

This way, you can be fairly secure in the ability to get more if you need it. Obviously care should be taken as to where you start/stop/repair or whatever -- but I'm gonna trust their computer controlled mixing a lot more than trying to duplicate a limited home-made mix. I would certainly trust it enough to attempt a blended repair, patch, or a single panel in need of a refresh.

Just my 2 cents for those looking to expand beyond the Tremclad/Rustoleum color chart.