OST changing colors to signal....

[Bumgrundle]

..... that it wants to be cleaned? 0,o
I noticed in the last 2 weeks that my Orange Shoulder Tang will flush from his shoulder tag, to his mouth into a shadowy black when he wants the Skunk cleaner shrimp to clean him. I'm still trying to get a vid of this or even a picture, it just has never lined up quite right.
Has anyone seen this? or have pics or vids? I googled and could find nothing. It's really cool tho =)

 

[Quah]

I sell coral to a guy who has a large Vlamingii Tang that flashes his stress colors to signal the cleaner shrimp.