Prior to 1.12 development versions, beds used to have a grey color on maps. Note that this is intended:
MC-73046. Now that beds are available in 16 different colors, this may be considered a bug. See attached screenshot.
Note that, just like colored wool, beds have been used to create map art with the only currently known survival alternative for beds in map art being cobwebs. If the behavior described in this ticket isn't working as intended, I'd like to respectfully propose some ways to lessen the impact the fix would cause:
1. Change all bed colors on maps except for red beds as those are currently the only ones used in map art
2. Change bed colors on maps, but keep the map color of the head half of the bed the current color to preserve beds as an alternative to cobwebs.
3. Either of the above AND add the ability to obtain and place all-sided large mushroom stem blocks in survival as those have the same color on maps that beds currently have (see screenshot)
I'd also like to request that if this behavior is to be changed, it should be changed before 1.12 is released so people don't keep relying on beds for their map art.