Needed: Paint Mix Sampler

I want to get some paint to fix a scratch on my car, or on my wall.  I want to go to the store and not buy a quart or even a pint:  I just want a half-ounce.  I’ll take it home and try it out.  If it’s not right, I’ll go back to the store and put another 50 cents in the automated paint mixing machine, type in the code for the exact shade (out of 16.2 million colors) that I tried last time, and then follow the software instructions, one step at a time, to (1) Lighten (2) Darken (3) Add more Red, (4) Add more Green, etc.