Why Captain-class escorts are frigates

Should Captain- and Colony-class ships be classed as patrol frigates or frigates?

They are best described as frigates because they were wartime anti-submarine vessels built for that role.

Mission and wartime context provide the primary classification test. Captain-class ships were designed during the Second World War for anti-submarine work, which makes frigate the clearest label. A ship's later secondary use does not erase the purpose for which it was built.