When I put strawberries onto a sponge covered in cream, how can I prevent the strawberries from discolouring the surrounding cream?
I'd suggest 'sealing' the cut side of the strawberries by dipping them in gelatin (or a vegetarian alternative. Or you could use white chocolate.
When you cut a strawberry its moisture starts to seep out, you either seal the strawberries or you absorb the moisture. You could seal them as @ElendilTheTall suggests, which should work. Alternatively you could absorb the moisture by putting the cut strawberries on a towel and gently squeezing them a bit, and/or coating them with some powdered sugar.