So my house is complete, and plans finally passed for the granny cottage at the far end.

I have a whole box of house wire (2,5mm and 1,5mm) from my previous house build and it should be enough for the granny cottage. Earth is fine, but the wire I have is black and red. Can this still be used? I think I'm sure I heard the electrician who did my place say he used blue / brown. I recently changed a plug at my friend's place (must be 5 years old) and it was black / red to live and neutral, so now I'm confused.

If 100% not allowed, I wont ask electrician to use it. If I can, that's a welcome saving.

It's 100m rolls of Aberdare house wire still sealed.