Well first: not all other plant cells are green. For instant, the petals of a flower can be many colors and they are plant cells. Roots and bark of trees are not green. Cells deep within the structure of plants are not green.
Second, not all onion cells are lacking in green color. The bulb of the onion lacks green, but the stem and leaves of the onion plant are green.
The reason the bulb is white, and not green is that it has no chlorophill. The bulb is under ground and used for storage of nutrients and not used for converting sunlight into food energy. The leaves and stem are green because they do have chlorophill in order to convert the sunlight into food.
The cells in the root or bulb of the onion grow below ground. Instead of havingchloroplasts, which make chlorophyll, like the leaves do, they contain something known as a protoplast. This is an undifferentiated chloroplast, and it does not make the green, light-responsive chemical. Studies have shown that if these cells are exposed to the right chemicals and light, that the protoplasts will become chloroplasts.