I have a box full of paints (sprays and bottles) I use. Rarely can I find the right colors already made up so like Charles I custom mix every one I do. I also keep an eye out when I go to the store for new colors that I can add to the collection that at least are a close start.
As for specific brands. I've used (in the little bottles) the hobby paint Testor's though the bottles are too small to do any number of cars (since I tend to do a number of cars a year for myself and clients) I've found larger containers of enamel hobby paints made for metal that I've picked up (a pale yellow I use often comes to mind) And like Charles have a good collection of rattle cans but they tend to take more room to store
For stamps I either use a ink pad (though these do not hold up as long) or spray onto a magazine page then daub, check and apply. Sure a number of people here have observed this practice at some show where I'm applying some inspector stampings for a car owner
One thing to always consider IMHO is what your going to be marking. If your doing a driveline or something that will take a fair amount of paint mix up a batch at least twice as large as you will need. In that way you will not run out (you'll never get a second batch to match) if you run short or need to correct it just a little