Background colour (red/blue) and sound (loud/quiet) were manipulated in a series of computer games. Players using a blue screen improved gradually over the session, while red screen players peaked midway and then deteriorated. A similar pattern for heart rate was found, suggesting that arousal was implicated in the effect. Sound alone had little impact, but the red/loud combination was associated with perceptions of excitement and playing well. The results suggest that the aura of a computer game may affect cognitive and physiological responses.