why the coloured plastids of red and brown algae are called chromatophores

Chromatophores are pigment-containing light-reflecting organelles in cells. In brown algae the pigment present is fucoxanthin. It masks the other pigments such as chlorophyll a, chlorophyll c, xanthophylls, and beta-carotene. In red algae the pigment present is Phycoerythrin which is red florescent in color and masks other pigments and imparts different shades of red to the algae.