Dulux sells colour to order paint down to 250 ml tins (about the size of a coffee mug), they cost less than £5 each - something of a loss leader for them I bet. I think they call them match pots, or tester pots.
I buy them all the time for painting terrain, basing, etc. because it's as cheap as paint gets, and you can get any colour you like. Want a colour the same as your shirt, just wear the shirt to the shop so they can scan it and make paint to match: brilliant resource.