It always depends on the choice of the opponent. If the damage caused by the opponents weapon is less than or equal to 80 ({50,80}) choose the yellow armour, for the other available choices ({100,150}) choose the green armour to minimize damage per attack.
Regarding the choice of weapon, always choose green to inflict maximum damage in a given amount of time.
So the best strategy is to go with a yellow armour and green weapon if one has no prior knowledge.
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?