Questions:
Does the later clarification suggest the original PCN location was insufficient?
No, because the PCN was served to your car, so you are assumed to know where your car is. For the later Notice to Owner it might not be OK, because the owner may not have been driving.
Does the delay beyond the stated 21 working days carry any relevance?
No, there is no statutory period to reply to informal reps. However, 56 days is max for replying to reps against a Notice to Owner
Is the incorrect vehicle colour of any significance when taken with the above?
None whatever