...was undoubtedly Captain Blue
(Note: Captain Blue is on the Left.)
I met him in the BBC Pebble Mill studios in Birmingham, 1990 - where he played a leading role in my one and only radio drama production.
I candidly admit it is a bit lame to have met Captain Blue, and not Captain Scarlet - but I maintain Blue's role was consistently under-rated, and that Scarlet (and of course the Angels) took away a lot of credit that was due to him.
BTW - without his uniform and under studio lights, Blue looked rather different:
I can only say that - while not exactly jovial or friendly - Blue was highly professional; and I was very grateful he took time away from saving the earth to play the part of Henry David Thoreau.