electro has a point.
Not that it'd really make a difference in your decision, but this is from bell's website.
Paint can react with helmet shell material and affect its protective capacity. Although the outer shell of the helmet is constructed of thermo set composite materials and finished with a polyurethane coating and although it can theorically be repainted with high quality air drying acrylic or polyurethane enamel, we do not recommend to have the helmet decorated by an independent painter. We often see helmets decorated by independent painters being damaged during the painting process. In no case should the helmet be dismantled during painting. Paint requiring heat curing should not be used. Paint penetrating the interior of the helmet can affect the performances of the helmet liner and other components.