The safest bet is not to paint the helmet... Depending on the paint, it can effect the plastic shell and damage the helmet (i.e. lessen protection).
In terms of the rules (talking ITU here, not ironman), there's a lot of grey area. Technically, painting consists of altering, so an official would be within their rights to block someone from using a painted helmet. a few other rules come into play.
Technically, from the cycling equipment section, only the manufacturer's logo can appear on your cycling equipment, no other "stickers" or logos are permitted.
But...
in Appendix F (Uniform rules):
The helmet can be any colour or design and include any logo provided it meets the requirements in B & c (which are b) helmet race stickers, provided by the ITU or LOC must be placed both on the front and both sides of the helmet without any alterations, no other stickers allowed, c) there must be clearance around official helmet number sticker of 1.5cm to ensure that the number is clearly visible).
So theoretically, a custom paint job could pass the muster as long as it was painted and not using decals, and afforded the necessary clearances. To make sure you're in the clear, you'd likely need the paint job to be completed by the manufacturer so that it wouldn't potentially affect the safety approval of the helmet. Home done paint jobs bring you into that grey area of alteration, and carries a heavier risk of not being allowed.
In terms of the rules (talking ITU here, not ironman), there's a lot of grey area. Technically, painting consists of altering, so an official would be within their rights to block someone from using a painted helmet. a few other rules come into play.
Technically, from the cycling equipment section, only the manufacturer's logo can appear on your cycling equipment, no other "stickers" or logos are permitted.
But...
in Appendix F (Uniform rules):
The helmet can be any colour or design and include any logo provided it meets the requirements in B & c (which are b) helmet race stickers, provided by the ITU or LOC must be placed both on the front and both sides of the helmet without any alterations, no other stickers allowed, c) there must be clearance around official helmet number sticker of 1.5cm to ensure that the number is clearly visible).
So theoretically, a custom paint job could pass the muster as long as it was painted and not using decals, and afforded the necessary clearances. To make sure you're in the clear, you'd likely need the paint job to be completed by the manufacturer so that it wouldn't potentially affect the safety approval of the helmet. Home done paint jobs bring you into that grey area of alteration, and carries a heavier risk of not being allowed.