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It turns out that chromatic number is easy to find.

sage: G = graphs(6)
sage: g =
sage: g.chromatic_number()

By the way, to find out what you can do with a graph you can do

sage: g.?

and a list of all methods will appear, including this one.

However, I believe the syntax for your other thing is

sage: g.independent_set(value_only=True)

This fact is hidden here in the documentation. Perhaps it should be a separate command if this is a common invariant?

Anyway, then you can use a basic filtered list comprehension to finish it off.

sage: [g for g in graphs(3) if g.chromatic_number()==g.independent_set(value_only=True)]
[Graph on 3 vertices, Graph on 3 vertices]