We often change paint colors by adding something else we have on hand. I have a gallon of a very powerful blue (think Michigan colors) that has already tinted 10 gallons of white and I've only used about an inch out of the gallon can. In general, you should be fine just be sure to stir the other can before sampling from it and only mix latex with latex. It doesn't have to be the same finish as you usually only add a small amount of the tint paint.

I start with an old piece of board or drywall and get some paint from each can and play with mixing fingerpaint until I see a color I like. Then I know what to expect when I start adding to my original can; just keeps me from getting surprised if the color goes a different direction that I expect. My partner tends to dump but I'm more cautious by nature, as well as less experienced.