It's perfectly valid for the function getColor() to return different value for each invocation, e.g., think std::rand().

However, the function as you wrote, when used in the expression (getColor() == getColor()), results in undefined behavior. This is because equality operator (==) is not a sequence point in C++, and thus by calling getColor() twice, you are modifying variables a & b more than once between a pair of sequence points (see "Undefined behavior" section here: http://en.cppreference.com/w/cpp/language/eval_order).

In general, you cannot be sure of the order of evaluation for a statement of form say, f() == g(). The relative order in which f & g are called is unspecified. Also, the behavior can easily become undefined if you violate certain conditions in f & g (as in the case above).