Rings, amulets and I believe weapon damage modifiers merge with druid shape. Armour doesnt & sheild doesnt as well as enhancement on weapons as far as I know.
You have it backwards: I can tell you that armor, shield and helmet properties merge with wildshape. Elemental damage on weapons merges, although its not shown on your character sheet, and enchantment bonus does not merge. Rings, amulets, bracers, and boots do not merge with wildshape. I really don't know why there are three colors, to answer the original question, though...