How could it be monochrome? The TV-am analogue clock started off yellow!
Or do you not mean "monochrome" as in "black and white"?
Monochrome = single colour - doesn't have to mean B&W... Green-screen and amber-screen computer terminals were 'monochrome'...
The TVam clock looks like it could conceivably have been a key-only source, filled with a matte (so it could have been any single colour) and possibly given a mixer edge or drop shadow.
I can tell you that they used CDL mixers, similar to the GVG300 (and that it was a lucky escape to have been turned down for a job there!)
CDLs were used in all three News studios at TVC - N1, N2 and N3, and a stretched one was also used in the BBC's large CMCCR OB truck (which was designed to combine the outputs of other OB trucks, and didn't have cameras of its own) in its 80s iteration.
The News CDLs had built-in synchronisers on each bank - allowing you to freeze, slide, shunt etc. and also coping with non-sync signals if you preset cut them.
The CDLs in News were in use right up until the move to TC7 (1997?) and N6
(1998? 1999?) when Sony 7350s were used. (The move to N6 was before the move to the beige and red look - they replicated the N2 'cut glass blue' look in N6 for a bit)