Odd that Google does not find the phrase "Any changes to the vehicles details must be notified to DVLA by law" (at the time of writing!).
Closest I can find is here: Gov.UK:
You must update the details on your registration certificate (V5C) to tell DVLA about:
•mistakes on your V5C
•most changes you make to your vehicle
Changes you need to update
You must update your V5C if you change any of the following:
•colour

If those pages follow the same format as the Highway Code, then anywhere that it says "you must" relates to a legal requirement.