What kind of cable is this?

Hi all, I'm in the process of replacing the 20 year old garden lights and the buried cable running to them is a black 4 core with core cable wires coloured blue, red, yellow and brown. The red, blue and yellow are clear (live, neutral and earth) but the brown is confusing me.

To give some context the lights are on a switch and on a sensor. I would have expected both switch and sensor cores to feed the light live but that doesn't seem to be the case even though they worked.

In the attached picture the brown and blue on left are for light bulb, green went to earth on light post. Core cables on the right come from mentioned underground cable and there are 2 of each core to join to next light.

I'm replacing existing cable with new 4 core SWA do just want to make sure the new colours match up with the guidelines while doing with old.