Minecraft wiki entry for Map_item_format, associated text: Blocks are colored according to their material. Each material has a base color which is multiplied by 135, 180, 220 or 255, and then divided by 255 to make the map color. Each base color below is associated with four map colors - to get the first map color ID for a base color, multiply the base color ID by 4. (Note: ID 0 has 4 gradients of itself despite them all being transparent)