Whenever you are deciding on a paint colour, it's important to consider other colours within the palette and the general style and feel you wish to create in the space. When painting the exterior door of a brick house, your primary constraint will be the dominant deep reddish colour of the brick. As brick homes can be one of a myriad architectural styles, the only constant is the earthy red tone of brick and the effect you wish to create, whether the door blends in or "pops."
For a front door that blends seamlessly with the house facade, find a paint colour that is highly similar to that of the brick. You might select a colour of the same family, with a slightly darker or lighter shade. The monochromatic facade has the advantage of being a "safe" colour scheme where you don't risk clashing colours. Look at variations in the colouring of individual bricks, and pick one that slightly stands out. Many paint suppliers can blend paint to match a small sample. Ask if they can match a chip of brick so that you can determine the precise colour. As brick colouring can range widely, a monochromatic door may have a deep, berry hue or almost a terra cotta orange.
An adjacent colour combination uses two colours that are located next to one another on the colour wheel. For example, blue goes with the adjacent colours blue violet or blue green. In the case of a brick house facade, a reddish-orange brick could be paired with a deep red door. Brick that has a deep wine or burgundy colour would combine well with a brighter, fire engine-red door. For adjacent colour combinations to look best, you should combine colours with slightly different shades or intensities. If the brick is somewhat washed-out, use a rich and intense colour for the door. If the brick is brand-new and saturated with colour, a more muted tone may work well for the door.
For a complementary colour scheme, two opposite colours are paired together. For example, red and green are located opposite one another on the colour wheel, as are yellow and violet or blue and orange. Complementary colour combinations are the riskiest and most delicate. It can help to select one dominant colour and one subtler one, thereby avoiding an impression of "clashing" between the opposite colours. As most brick houses are somewhere between red and orange, complementary doors are either blue or green. Bright blue doors add a measure of cheer to otherwise staid brick exteriors. Kelly green or forest green both give a somewhat traditional, yet striking, contrast to brickwork.