The GREEN one is your residency which you need for your driving license and under spanish law this should be changed if you live here for more than 183 days, or if you live here full time.
It's not necessary to change your licence if you live here full time or more than 183 days. If its a photocard licence you can use it until it expires, when, if you are here more than 185 days a year, you can only renew it in Spain, not the UK. If it's a paper only copy, then you must exchange it within 2 years.
This is a EU requirement and, for example, EU citizens living in UK have to sign on this register there as well.
It is allowed under the Eu Directive on freedom of movement to require EU citizens to register if they are here more than 90 days, BUT it is not a requirement in the UK (yet)