I bought some SWA cable to use in the garden. It has three 2.5
mm2 cores, with brown, black and grey insulation.

The label doesn't say which colour to use for what but it does
have an new/old conversion chart.

new old
brown red
grey yellow
black blue

I think brown is live, but I'm not sure what to do with the
others, because I've only used twin and earth until now.

Which one is what?