He'd always been fond of blue. The japoor he'd carved for her, the highlights of his astromech, the detailing on his starfighter had all been blue. When the men chose blue for their colors, he'd smiled. His eyes had always focused more on the blue of his padawan instead of the orange of her skin.
Having a color stolen was so trivial, but when the helmet and its red vision were gone, so he could see Luke's blue eyes peering down, it lanced through Vader again. Blue was his color, as memory of the red faded away with his life.