beacons turn black in the end

beacons turn black in tehe end 

What I expected to happen was...:
i wanted the beacon to be white as normal

What actually happened was...:
the beacon turned black gradually when going up

Steps to Reproduce:
1. Place down a beacon in the end 

2. wait for the beam of the beacon

3. follow the beam upwards 

4. the beacon beam will be turning black