On the entries I have viewed green ink is used for a change of name other than change by marriage.
By this I mean the change was notified by a doctor when the person joined a new practice or by hospital admission, etc.
Changes of name by marriage seem to be noted by the use of blue ink (though I have seen at least one page where the whole page was completed in blue ink in such cases a mixture of green ink and black ink is used for changes of name and even corrections)
Red ink seems to be reserved for correction of errors.
The alterations would have been done by both the ID Registration authorities (until 1952) and by the NHS from 1948 until 1991 when they changed to a computerised system.
NHS changes are usually in green ink and marriages have CR283 and a date nearby, IDR changes may be in one of several colours (I've seen black, blue and red) and usually have a date, a three letter ED code plus M nearby.
CR283 is a form that is used when there is a Change of Surname, Forename, or Date of Birth
CR283 is a form that is used when there is a Change of Surname, Forename, or Date of Birth
Cheers
Guy