The (normally green) marker that represents a map in an item frame on a map is white if the map in the item frame is outside the area that is covered by the map. This is confusing if you have a copy of that map in your inventory because it looks like there is another player holding this map.
- Create a map
- Move outside the area which is visible on the map (your marker changes to a circle)
- Place the map in an item frame
→ The map marker which should be green is white
The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.
The reason for this is that the method net.minecraft.world.storage.MapData.updateDecorations(int, World, String, double, double, double) uses always the white point if a marker is outside of the map (type 6).