Answer:
Due to different pigments.
Step-by-step explanation:
The multicellular algae are red, green and brown because of the presence of pigments. The red algae are red in colour because of the presence of the phycoerythrin which is a type of pigment. This pigment reflects red light due to already presence in its body and absorbs blue light that is necessary for photosynthesis process. The brown algae contain fucoxanthin pigment and green algae contain xanthophylls pigments that are responsible for its colour so we can conclude that these different types of pigments are responsible for the different colour of algae.