In motorsport, it is the norm for drivers to customise their crash helmets; mostly to express their personality but sometimes to advertise their sponsors too.
Bobsleigh has some similarities with motor sports, not least the requirement to wear crash helmets. Unlike luge and skeleton (which both use specially designed helmets) bobsleigh appears to be use bike/car racing helmets. For the most part, bobsleigh drivers appear to simply race with a "stock" helmet showing no more than the helmet manufacturer or plain white - i.e. no personalisation. Where a design or pattern is used, this design seems to be adopted by both the driver and the brakeman (so cannot be considered "personal").
Given that the sleds and the competitors' lycra suits are often customised (i.e. not just off the shelf), is there any particular reason for helmets not being customised too? Is there a rule that prevents this, for example? (Ben commented below to point out that this is not the case - helmets sometimes adopt these team colours too).
To be clear - I am asking why personal helmets aren't used and my question isn't just limited to Olympic competitions.