I encountered a zombie brandishing a grey lily pad in release 13w03a.
(See also old bugs MC-222 and MC-2593)
May also apply to other third parties (players, skeletons) carrying ferns, grass or other biome-coloured items.
Colour of an item when picked up by mob
Spawn egg reised by zombie is white (no texture)
Spawn Eggs and Firework Stars are pure white when held by zombies
Vines, Fern, Grass and Lily Pad appear grey when held in F5 mode