A characteristic brown pigment called fucoxanthin gives brown colour to Phaeophyceae, like the Sargassum. They vary in colour from olive green to various shades of brown depending upon the amount of fucoxanthin present in them.
The members of Rhodophyceae, like Gelidium, have both chlorophyll a & d but they also contain the red colored pigment, phycoerythrin, which in turn causes red color to reflect back in the visible range when light shines on these algae.
The members of Chlorophyceae, like Spirogyra, have chlorophyll a, chlorophyll b and carotenoids. They resemble the green land plants.