The bug

When coloring a beacon beam with colored glass, the color will be correctly applied only if you place the colored glass directly on top of the beacon. If you place the colored glass a few blocks above the beacon, the beacon beam will act as if there were white glass blocks between that glass block and the beacon.

How to reproduce

1. Build a beacon
2. Place a red glass block on top of it
   → The beacon beam is completely red
3. Place another red glass block on top of the first one
4. Break the previous red glass block
   → The beacon beam is now pink-ish