Most supermarkets carry white buttercream these days. The yellow in the butter is usually an added color for commercial purposes. The simplest trick to making buttercream white is to whip the butter at the start before adding any other ingredients. This lightens the butter and makes it white. Another tick is to add the tiniest bit of purple gel food color. This is a trick used by painters when they color our homes. The slightest bit of purple added to white will make the white whiter. (purple added to white reduces the yellow)