You need a white light!!!
It can either be an all round light when underway (which is what an anchor light is) or a stern light (which should be visable up to 22.5 deg abaft the beam)
Many rec boaties seem to get this confused and think you only need sidelights when underway.
Q.How can you see only side lights when coming up on the stern of another vessel in darkness?
A.You can't!
When at anchor you need the all round light on, sidelights off.
Basic wording I know, but with no all round light or sternlight on, don't be surprised to see a bow come crashing through your stern.