old colours are still allowed until april 2006

technically you aren't supposed to mix colours in a job BUT you are allowed to add wiring in one set of colours to existing wiring in the other and someone inspecting in future would have ni idea if it was installed at the same time or not

sleeving is preffered to tape as tape has a tenency to loose stickyness with age and come off whereas sleeving can't come off as long as the wire remains in the terminal

red-red (with old colours) and brown-brown (with new colours) T&E does exist but tends to be more expensive and is rarely used.