Pink, Grey, Red, Black Wool flickers White when pushed with piston

Pink, Grey, Red and Black Wool changes colour to white for a split second during a piston push or pull

Steps to reproduce:
1. Place a sticky piston beside a lever
2. place pink, grey, red or black wool on the piston head
3. Fire piston on and off
4. Notice the flash of white between extensions

Not sure why this doesn't affect the other colours.