**The Free On-line Dictionary of Computing (30 December 2018):**

graph colouring
graph coloring
A constraint-satisfaction problem often used
as a test case in research, which also turns out to be
equivalent to certain real-world problems (e.g. register
allocation). Given a connected graph and a fixed number of
colours, the problem is to assign a colour to each node,
subject to the constraint that any two connected nodes cannot
be assigned the same colour. This is an example of an
NP-complete problem.
See also four colour map theorem.