I'm wondering what would be good things to add to resin to make it tougher. Something like JB Weld, but using my existing bisphenol based epoxy, bought in bulk?

Just remember that some additives will influence the colour quite a lot.  eg fumed silica in any quantity turns the mix opaque white.  Milled carbon turns it charcoal black.  Glass in larger quantities will turn it a bit of a semi-translucent murky grey colour. 

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant