Batman has his own system of color, inscrutable to most humans.

Batman sees color like a bat. Not color blind per say but definitely different than us.

Maybe Batman observes the Sabbath?

The inclusion of Indigo was hotly debated for a few hundred years, apparently not everyone can see 7 colors in the rainbow, only 6. It’s a (mildly) interesting story worth looking into. As for the order, yeah he really fucked up