"You are making an assumption that the lower MW truly is safer" Again inequitable conduct defense. Nothing to do with obviousness.
"If I patent a blue wonder hammer, then later try to extend the term by patenting a pink version with the claim that it strikes harder, we have an issue. "
Not obviousness defense IMHO.
Obviousness would be if somewhere else in the prior art it was disclosed "change in colors may lead to higher impact"
So a finding that "hammer with pink color strikes harder" would be an obvious finding based on prior art