I should probably advise you if you're not qualified to be very careful, esp. with 400v 3ph supplies. And make sure you're not breaking the law.
To answer the question - your single phase cct should be wired with Brown live, regardless of which phase it's connected to.
Be VERY careful if your connecting into a 3 phase supply with the old colours (red, yellow, blue for live and neutral black), as your new wiring will have blue as neutral and the existing wiring has blue as live - get it wrong and you'll have 400v instead of 230v!
mab2