The emerald don't have color skin on the map

This might not be a bug, all ore blocks (gold blocks, iron blocks, diamond blocks) are grey on the map. That said, I'd love to have more colours on the map 😛

Yeah more colours would be nice, sandstone looks grey as well.

This is intentional. like Daedalus said diamond blocks iron blocks and gold blocks look the same way on maps.

This does not only happen for emerald blocks. Most blocks are rendered as grey on the map. Also colored wool is always white.

sandstone is also grey. and, indeed, it's not a "bug", but it's irritating.

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This is working as intended, see http://www.minecraftwiki.net/wiki/Map_item_format