I think if you paint or put stickers on a helmet it invalidates the CE homologation. So perhaps not a good idea for your bike lid. Fine for M3W where its not compulsory wear & is more for looks & weather protection.
Actually race car drivers have their helmets painted all the time, and I see stickers a lot. I don't think it influences the safety qualification, but then I am in the US, so it might be differnet eleswhere.
