The word kasaya means dull, muddied or off colour, and that is the specification. The Vinaya directs that a monastic robe be dyed with one of a small set of permitted substances and expressly not left in any of the pure or attractive colours, so the robe is defined negatively: whatever it is, it must not be a good colour.
The original cloth was pamsukula, dust heap rags, gathered from what had been discarded, including shrouds from charnel grounds, then washed, cut, pieced and dyed. Donated cloth was accepted early on and became the norm, but the pieced construction was retained deliberately, and the reason given in the tradition is that a robe assembled from strips of cut cloth has no resale value and offers nothing to vanity.
The dyes were vegetable and mineral: heartwood, bark and root, with jackfruit heartwood the classic source in South and Southeast Asia, producing the range of ochres, saffrons and browns still associated with Theravada communities.
The resulting palette now maps the schools rather than the rule. Theravada monastics in Thailand, Sri Lanka, Myanmar and Cambodia wear ochre through to a deep saffron, with forest traditions tending darker. Tibetan monastics wear maroon with yellow for formal occasions. Chinese and Korean communities wear grey or brown for daily wear with a formal outer robe over it. Japanese priests wear black, with colours by rank and occasion in some schools.
The kathina ceremony, held at the end of the rains retreat, is the annual occasion on which lay supporters offer cloth to the community, and in its strict form the cloth is cut, sewn and dyed into a robe within a single day by the assembled monastics.