With the 'armorname' attribute, you can set the prefix of your armortextures.
All values are valid. You have to name your armortextures e.g. dirt_1.png (for legs) and dirt_2.png (for helmet, body and boots) and set 'armorname=dirt' and NOT 'armorname=dirt_1.png'.
Default value: 'gold'
armorname="dirt";
armorname="redstone";