The guidance is here
www.gov.uk/guidance/working-safely-during-coronavirus-covid-19/close-contact-services
It doesn't say what the media articles are purporting that it does.
I suspect the hairdressers who are commenting have not read the updated guidance
All it says under the 'mitigating actions section' is
"keeping the activity time involved as short as possible"
So if it's a six hour treatment, don't take six and a half.
It doesn't say that treatments must not be longer than a specific cut off time.
People of any colour can have long treatments