Can color crystals be re'd for augmentation slot components? Or is my bank full of color crystals completely useless, given the availability of cartel market crystals?
No, they don't give augmentation slot components. Only items that can be augmented will give the components (ie a lightsaber or offhand for an Artificer).
I wouldn't say your bank full of color crystals is useless either. Craft & sell the base +41 color crystals at semi-low prices. You won't make a huge "profit" but you can at least convert your inventory into credits.