Plant a Rainbow

Read the book Planting a Rainbow, then plant your own! All you need are seeds of various colored plants and a garden plot.

Here is a list of  flowers that are all summer-blooming annuals that will bloom well in all climate zones via How Stuff Works:

Dianthus, petunia, verbena, and zinnia produce red or pink blooms

Calendula, marigold, and zinnia produce orange or yellow blooms

Nonflowering, leafy green plant, such as mint, for the color green

Dianthus, petunia, verbena, and zinnia produce blue or purple blooms