Shiny materials are gotten by summoning. When you turn the entire field one color and can summon the enemies will leave behind one of the shiny materials depending on what color you summoned.

For instance, summoning saints will get you shiny salt, while summoning grim reaper will get you shiny soot.

If it takes too long, just beat the game and you'll have FieldBlue, FieldRed, etc. on your new game + which will make things considerably easier. There is no FieldWhite or FieldBlack, so you'll just have to work around that.

I would just get enough to forge three rainbow weapons then assemble and disassemble if you decide to use new party members. You could make three rainbow armors as well, but I prefer the color armor you get from stealing from the dragons.