Red bristles identify our softest bristle brush heads designed for first-time users and for those with shorter teeth and/or sensitive gums.   We recommend this brush for first time users because of the unique reciprocating brush head movement.  You’ve probably never brushed in this manner before and this brush head allows your gums to become acclimated to this type of effective brushing movement.   First time users can graduate to a Standard Soft Brush Head within a week or 2 if desired.  Smaller than Extra Reach Ultra Soft Brush Head.