I'm aware that not all plant cells are green but why? I'm not really sure about the reasoning behind the fact that not all plant cells are green.
Most of the plant cells are green, due to the presence of chloroplast- a plasmid which contains chlorophyll (a green colored pigment that carries out photosynthesis). These green cells help in the photosynthesis process and generate energy and organic molecules for maintenance and further growth.
However, there are cells that do no participate in the photosynthesis process and hence do not posses chloroplast. Examples of such cells and plant parts are roots and/or bulbs, flower petals, many fruits, etc. The roots or bulbs of various plants contain non-green cells that are used for storage of nutrients and not for participation in photosynthesis. An example is the onion bulb.
In fact, flowers do contain chloroplast even if they are colored. The photosynthetic activity continues till just before the flower open up and it falls afterwards. The color pigments simply mask the chloroplasts and are need for attracting the pollinators.
hope this helps.