I wonder why the washing time would be very different.
The strength of these liquid fixers is pretty much the same. They are nearly entirely ammonium thiosulphate and they have about the same specific gravity (=concentration). The other ingredients are sulphite, metabisulphite etc.
The pH of the colour fixers is close to neutral (Kodak just a little on the acid side, Agfa and Fuji just on the alkaline side) so I wouldn't expect much difference.
FWIW, Agfa in Australia (just before their demise) started labelling Universal Fixer (i.e. colour fixer) with dilutions for black and white. Copy attached.