but it seems that some CSS syntax only allows for three-characters (i.e. a reduced set comprising just 4096 colours).

That doesn’t sound correct. CSS should/will accept any valid color codes. Where are you trying to put that in?

Embarrassingly, I was being a bit stupid. The example code I picked up was only using three character colour codes – so I assumed that this was a limitation. I now realise that if I enter the full six characters, they are accepted.

Forget I raised it – but many thanks for answering!