Brown is the colour of earth, wood and skin, of soil and bark and autumn leaves, and it is not a spectral colour at all but a dark, low-saturation orange, a hue that exists only in relation to brighter surroundings. Its symbolism is grounded, literally, in the earth: it stands for stability, the humble, the natural and the down-to-earth, and by extension for the rustic and the poor.
In the Christian tradition brown is pre-eminently the colour of monastic humility, the undyed or cheaply dyed wool of the Franciscan and Capuchin habit, chosen precisely because it was the colour of the cheapest cloth and thus of poverty embraced. That association gives brown a dignity within religious dress that its everyday plainness might not suggest. Brown carries senses of reliability, warmth and comfort, but also of dullness and the drab. It has little formal liturgical use beyond the penitential and the monastic, and in the modern colour literature it is read as the most earthbound and grounding of hues, tied to the physical and the material.