EDIT: The deleted comment this responds to claimed that you should never use the blue armor, because against any weapon, either the green or the yellow armor is better.
It can make sense to choose the armor that is not optimal in any case if it is good enough in more cases. There are no bonus points for winning by a larger margin.
Note: this image does not belong to me; I found it on 4chan. It presents an interesting exercise, though, so I'm posting it here for the enjoyment of the Less Wrong community.
For the sake of this thought experiment, assume that all characters have the same amount of HP, which is sufficiently large that random effects can be treated as being equal to their expected values. There are no NPC monsters, critical hits, or other mechanics; gameplay consists of two PCs getting into a duel, and fighting until one or the other loses. The winner is fully healed afterwards.
Which sword and armor combination do you choose, and why?