A fn to check if a node is trivially colorable For graphs who's color classes are disjoint then a node is 'trivially colorable' when it has less neighbors and exclusions than available colors for that node.
For graph's who's color classes overlap, ie some colors alias other colors, then this can be a bit more tricky. There is a general way to calculate this, but it's likely be too slow for use in the code. The coloring algorithm takes a canned function which can be optimised by the user to be specific to the specific graph being colored.
for details, see "A Generalised Algorithm for Graph-Coloring Register Allocation" Smith, Ramsey, Holloway - PLDI 2004.