Matching paint colour to brick colour can be challenging if the brick is made up of different subtle variations in colour. With tens of thousands of available paint colours, plus the ability to custom-match paint to brick, it won't be difficult for you to find a perfect match, even if your brick contains several different shades of colour. The staff at a good paint store can also help by colour-matching the paint perfectly, if you can bring them a sample of your brick.
Take a brick or a piece of the brick to a paint store, and ask the staff person to match the colour with the type or sheen of paint you plan to use. A whole brick will be easiest to match, but a good paint store can get a good match with a piece as small as 1 inch square.
Pick up as many paint swatches as you can at the store, if you can't bring in a sample of your brick. Ask the staff for larger swatches, which many paint stores keep behind the counter for their professional painting and decorating clients.
Tape the swatches that seem to match the brick colour the best directly onto the brick. When matching exterior brick, do it on a bright, sunny day. When matching interior brick, add light sources like lamps to get the most light possible.
Stand back at least 10 feet. Even though some of the colours may be very similar, you'll find that some blend in better than others. Look for the swatch that seems to "disappear" against the brick facing.
Purchase a sample quart or 56.7gr tester of the colour you've chosen, and paint a test patch near the brick to make absolutely sure it's the colour you want. If it's "off," ask the paint store to tweak the colour or hue.
For the greatest selection of paint swatches, go to a home improvement store where they sell several brands of paint. If you find one you like, you are not limited to that brand; paint stores match colours across brands. Most brick has a fairly flat or matt finish. Choose a gloss level that most closely matches your brick.