It would depend on whether the bumper is being painted the same colour or a different colour and will also depend on the colour itself. For a complete colour change including the prep, paint and materials, you'd be looking at around £150. This could vary depending on how busy the paintshop is at the time and whether they have any 'left over' paint in the correct colour. If the paintshop is busy, booth and oven time will be at premium so they will likely charge more, likewise if they're struggling for work or someone there is willing to do the job on the side it'll probably be less.