Minecraft – How to detect if a player is standing on a certain colour wool

This answer was written prior to the release of 1.13 and "The Flattening" that came with it. An updated answer can be found here.

You're actually pretty close, and definitely on the right track. Instead of {Inventory:[{id:<block ID>,data:<data value>}]}, use {Inventory:[{id:<block ID>,Damage:<data value>s}]}. The s after the data value is important since it's stored as a short, not as an int. (As an aside, if it were stored as a byte, you would append a b instead.)

It will succesfully run the command, but when you check the commandblock you aren't standing above a wool block, so it says '[21:28:27] Failed to execute 'detect' as Heroblind55246'. Try to do something like this:

/execute @a ~ ~ ~ detect ~ ~-4 ~ minecraft:wool 15 say hi

I hope it will work!