The Bug:

minecraft.used:minecraft.COLOR_dye doesn't increase when applying a dye to a sign.

Steps to Reproduce:

• Create a scoreboard objective for tracking the use of any dye and set it to display on the sidebar.

/scoreboard objectives add UseRedDye minecraft.used:minecraft.red_dye
/scoreboard objectives setdisplay sidebar UseRedDye

• Right-click on a sign whilst holding the dye of your choice.
  →  Notice how the scoreboard doesn't increase.

Information:

This issue works for all dyes.