This really depends on the person. I like to overdress a bit, I'll always err on a being a bit too warm over a bit too cold. 40-50 I just wear a cycling cap, I have stacks of them. 25-40, I like Smartwool's thin/tight running beanies under the helmet + a wool neck gaiter. Below 25, I go with a balaclava or even a ski helmet (if it's really cold).