Just an aside, when I worked at Home Depot a couple of years ago, the color dye ratios is actually printed on a label affixed to the paint can when you get it mixed. You can also request the label w/o the paint. In other words, you can have them scan a color chip, print up the label and you can just take the label with you.
Unforunately, I didn't work in the paint department, so that's the extent of my knowledge on the topic.