An Idea:
What you could do is simply start with a set of armor thats base painted to a colro of your licking, like white or silver, probably good choices. Then you can do tie dyeing on regular cloth or purchase pre-tie dyed cloth or shirts, etc. The trick is to get the TD pattern and colors onto the armor right?
Brush the armor with clear casting resin or spray fixative (usually found at craft stores like Michaels).
Pull or place the cloth over the armor peices, smoothing it out to the armor. around the back of the armor pull the cloth around and hotglue the cloth to the back.
Now to get fancier, and for other painting purposes (weathering a tye dye!), get thick laminating resin/epoxy (also found at craft stores). It will stick and fill the gaps in the cloth and level to a smooth surface that is crystal clear, then you will have a harder/smoother/durable surface to work with.
For the craggy peices like the helmet and the guantlets, you should probably just go with a fixative so you can life the cloth and place it down and make cuts/folds to it as needed in order to accomodate all the undercuts and details.
Another method:
Using a computer or overhead projector, project an image of tie dye onto the wall, hang up your armor and mark the patterns and colors with matching color sharpies and simply paint on the colors and patterns afterward. Just be careful tnot to mark or paint your wall.