Solution:
The presence of pigments that give algae its conventional colour is the primary criterion for categorization.
| Name of the pigment | Color imparted | Chlorophyll type |
| Chlorophyceae | Green | Chlorophyll a and chlorophyll b |
| Phaeophyceae | Brown | chlorophyll a, chlorophyll c, fuxoxanthin |
| Rhodophyceae | Red | chlorophyll a and chlorophyll d, phycoerythrin |
Now, algae are classified on different criteria such as:
(i) Flagellation
(ii) Storage products
(iii) Cellular organization
(iv) Constitution of cell walls