Pure speculation but found a site online "Headdress: A pictorial history of military headgear from 1660 to 1914."
http://www.reenactor.ru/ARH/PDF/Rankin.pdf
It mentions a French Officer's shako with a 'red woollen ball' (circa 1812) and indicates that perhaps the colour may be for different companies.
Photo #36 :British officer's shako , 76th Foot ,1835-44.
"Other ranks wore an upright worsted plume until 1835 ,when all ranks adopted a ball , white over red for the Line , White for Grenadiers , Green for Light Infantry and Black for The Rifles."
Could the 'woolen ball' have evolved into a boss ?