Steps to reproduce

1. Place a sign and input some text on it.
2. Use bonemeal on the sign.
3. Use cocoa beans on the sign.
4. Use lapis lazuli on the sign.
5. Use an ink sac on the sign.

Expected result

Same effect in each of steps (2) - (4) Either you can dye signs with these items, or you cannot.

Observed result

Bonemeal, cocoa beans, and lapis lazuli work can dye a signs, but ink sacs cannot.