Confirmed with the uploader and sheet maker (Techokami) that the Green Emerald on the far right was a custom sprite, and doesn't actually exist in the game anywhere. During the opening cutscene, the palette loaded in includes two dark green shades, both used for the grass and trees; not the Emeralds that Knuckles collects. Those shades on the palette were applied to the Emerald sprite, and boom, Green Emerald that never actually exists anywhere in the game. The only Emeralds that actually appear in Triple Trouble are the five colors on the left of this sheet, plus a lighter yellow one at the very end of the game. Never a green one.

Is "Knuckles T15" an old screen name for Techokami?

Why does this sheet have things that I think aren't related to Knuckles?

This sheet is missing the sprites used at the end of ADZ3.