When Captain Black calls, you see two green rings.

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Hmmmgood point. What about Capt Greywhat colour button would he get?
You're not shown the desk buttons very often, so I think this wasn't worked out very well. I do know in one episode Cpt Scarlet calls Destiny Angel and his shoulder lights flash white, and I wondered how he would tell the difference between Col White or one of the Angels calling him.
I also found how they called one another a little strange. There is never any preliminary "Scarlet here" or anything. They just activate their cap mike (or Col White his desk buttons) and start talking, assuming the person they are calling is ready to listen to them. What if someone doesn't have his cap on, or White is away from his desk?
And I don't think there is a Capt Yellow in the original series. Which is kinda strange since it is a primary colour (although Ochre is kinda close).

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