Indigo jeans look blue. Which light is the dye mostly taking away?

Answer: Orange light wavelengths

Blue light wavelengths — Not quite. A blue-looking material is usually sending blue-rich light back to your eyes, not absorbing mostly blue. The useful color-wheel trick runs opposite your first guess: the seen color is often what remains after another band is removed. Indigo's chromophore absorbs around the orange part of the spectrum, leaving a purplish blue impression.

Orange light wavelengths ✓ — Right. ChemMatters gives indigo's absorption in the orange region, around 613 nanometers, so the light that reaches your eye is biased toward the complementary blue-purple. This is why "blue dye" is a slightly misleading everyday phrase. The molecule is not adding blue light; it is subtracting orange light from white light.

Violet light wavelengths — Not quite. Violet is close to the color family people associate with indigo, but it is not the main absorbed band cited for the dye. If violet were mostly removed, the remaining reflected light would shift away from the blue-purple look. Indigo looks blue because its molecule removes orange-region light more strongly.