You can still vote third party candidate for President if you're in a non-purple blue state. Electoral politicians are a cynical group. It's appropriate to approach elections with the same cynicism.

There are significant financial and regulatory barriers to participating in elections as a third party. In several states, once they've received enough of the vote during an election cycle, they have fewer hurdles to jump to get on the ballot during the next cycle.

Voting for either third party is effective for this tactic. A strong 'Libertarian' Party candidate on the ballot means that the vote for Republicans is split between two parties with similar aims. My favorite candidate from this group is Vermin Supreme. A vote for the Green Party does the same for the Democrats, but forces the centrist party to shift their policies to the left to prevent otherwise Green Party voters from going to the third party.

As always, voting is not a long term strategy for change, and should not take the place of direct action, street protest, labor organizing, or human rights activism. Building actual power is the only way to achieve a more egalitarian world.