What's the best method of painting over strong colours? I'm currently redecorating a buy-to-let flat that has garish green and red walls, which are a still showing through after three coats of paint.

First paint over the colour with a grey undercoat. This deadens the colour and stops it coming through when you paint your final colour choice. Another factor to consider is that most DIY paints can be a little thin in their consistency, and therefore coverage, so it's worth spending a bit extra to buy professional paint. One of the best white emulsions around is made by JW Bollom.