India ink is pigmented, not colored. I know; spent to much time over the years cleaning the crud out of drafting pens! He may wind up with a canopy that looks like it was painted with very find pigment paint. I'd recommend trying it on some clear scrap first. It might work fine, I'm not sure.
Another alternative to try is to mix red, blue, and green food coloring in equal parts. That should give a very dark gray color that is almost black.