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.