The cassock is the working dress of the Christian clergy rather than a vestment, and its whole grammar is worn in its colour.
Not a vestment
The cassock is a full-length close-fitting robe worn as ordinary clerical dress and put on under the vestments at a service, but it is not itself a vestment and carries no liturgical role of its own. It is what a priest wears about the church and the parish, and in choir it is worn by servers and choristers who are not clergy at all, which is one reason it cannot be a mark of orders.
Reading the colour
Where the cassock does carry meaning it does so through colour, and the reading is exact in the Roman Church: black for a priest, black with coloured trim and a coloured sash for higher ranks, purple for a bishop, scarlet for a cardinal, white for the pope. The Anglican and Orthodox churches keep the garment with their own conventions, the Orthodox outer cassock being wider and worn over an inner one. The shape has proved durable precisely because it is street dress for a clergy that also needs to be recognisable, and the argument over whether clergy should wear it in public is the same argument attached to the clerical collar.