Things that are colorful are brightly colored or have many different colors. A brilliant red cardinal is a colorful bird, while a brown sparrow isn't quite as colorful.
Some nutrition experts recommend eating a colorful diet, including vegetables and fruits of varied colors for optimum health. A colorful bouquet of flowers would be equally varied, perhaps with white daisies, pink roses, blue violets, and yellow carnations. Brilliantly colored things are colorful too, like a tree with orange leaves in the fall. Figuratively, colorful language is rude or coarse, and a colorful group of friends is interesting and varied.