So if im right in understanding what your saying ... I have a "Birchen Dorking" no silver needed to describe it ?
Yes Silver is not necessarily used to describe it because Birchin is typically used to describe the Silver Birchin color so really the Silver birchin part is not needed, but like Sonoran said, there is a gold birchin counter part that is called Brown Red in most breeds ( like Gamefowl) and Black Copper ( in Marans)
So Birchin is a color pattern, not a breed.
That being said, no I'm pretty sure that the Birchin color does not exist in Dorkings, Silver Grey Dorkings are genetically colored Silver Duckwing, not Birchin ( BTW in the case of describing Duckwing colors, you have to say Silver, Gold, or Red because there are those three base colors. )
So Dorkings are the breed with 5 toes, the Silver colored ones are named Silver Grey Dorkings but there genetic color name is Silver Duckwing, and you bird looks birchin, not silver duckwing, so I would say he is not a pure Dorking.