Strange behaviour and colour change!

[AVVITT]

Hi people
I was just watching my tangs tear apart a peice of floating seaweed then suddenly my white cheek started acting really strange in front of the mirrored section at one end of the tank.
The strangest thing is, she changed her shade of colour quite quickly from very dark grey/black to very pale grey and you could see it exposed some yellow lines on the back half of her body, a bit like my sailfin.

We were all so stunned as it was over a few seconds and I didn't get change to grab my phone in time.
I managed to get a video of her behaviour but she'd gone back to her normal colour by then.

Any ideas on what was happening?

 

[Timfish]

Display behaviour, some fish can dramaticly change their color when they percieve a threat or suddne upset.