Nathan is spot on about trying to go by color still in the unopened bottle,,,,,,but, there is one more thing involved
There was an FSM article a few years ago,,,,,,,I think it was Paul Boyer, but, am not sure,,,,,he painted a chip and took a photo right after it was painted,,,,,,then took another photo a day or two later, after the paint had full cured, and all the pigments had "migrated" for want of a better word
it can be a lot like the old "rubbing out the color" days with 1/1 scale auto lacquers,,,,,it takes some time for the paint to dry, and that is your true color that you will see on the model from that bottle
if you chip paints (I do a lot of that, old habit),,,,you don't want to do that onto a standard index card, either,,,,,if you don't want to use a plastic sheet cut into squares, at least use an Avery white label type of material, and stick that onto your card,,,,,,,,,index cards used straight will absorb some of the paint, possible changing the color you see, making it not match the model when you paint it on there
Rex