Holding items that can change their colors based on biome appear gray on the top side of their texture. This includes Grass Blocks, Acacia Leaves, Dark Oak Leaves, Jungle Leaves (which do have their fruits colored yellow), Birch Leaves, Spruce Leaves, and Oak Leaves. This is probably a coding oversight based on the fact that these blocks do not have their texture colored in the game files.
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Duplicate of MC-161841.