User-agent:
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.12) Gecko/20101027 Thunderbird/3.1.6
The Mesh::color function recomputes the colouring every time it called.
Can we check if it's already been computed? The problem though is that a
different type of colouring might be requested.
