It seems to me that it is very difficult to get an FAA second-class medical certificate if you are colour blind and whatever UK ratings/licence were held, can I suggest the colour blindness is a red(?) herring?
No it's not. You are just restricted to DAY VFR jus the same as a Class 2 in the UK is restricted. It is a fact that the pilot was unable to fly at night due to colour blindness.
Nope, there is no such as a 61.75 CPL. The 61.75 purely gives PPL privileges based on whatever the underlying licence states. This is made VERY clear by the FSDO when collecting the certificate. You can add ratings to it. I hold both a 61.75 and full FAA CPL/IR and I added ratings to the 61.75 before realising I was building it on a house of cards and went for the full certificate.