The name cauliflower is originally derived from the Latin word caulis. The meaning of this Latin word can be translated in several ways, the most common of which is "cabbage," though other translations suggest "stem of a plant;" the "flower," however, is the white part of the vegetable. A cauliflower is the same species as broccoli, except the flowers are tightly compacted and never open, unlike broccoli, which if left to grow often has purple flowers. Cauliflowers, when freshly picked, are very white, but as they age, the color turns to cream, and brown specs start to appear as the flower dies.
The reason a cauliflower is white is due to its chemistry, or more correctly, a lack of chemical reaction. The white flowers never open, and the thick leaves that surround the flower protect it from sunlight. Without sunlight, the flower cannot form chlorophyll, the chemical reaction that turns plants green. This results in an inability to change color and therefore it remains white.
Many grocery stores strip off the outer leaves from the cauliflower so that you can see the size and whiteness of the flower. From a presentational angle, this makes the cauliflower look better; but it also means that the whiteness will lose its color more quickly, because it has been exposed to light. Because it's been cut from the stem, the flowers will then start to die and turn brown. You should not purchase cauliflower that doesn't appear white, because this means it isn't fresh.
To keep a cauliflower white (and therefore fresh), the way you store it is important. Exposure to light will make it turn brown quickly. Don't keep a cauliflower in a sealed plastic bag, as moisture will form and make the whiteness fade. Put the cauliflower in the vegetable section of a fridge, ensuring the flower is upright and no other vegetable is touching the flower. You can expect most freshly cut cauliflowers to remain white in a fridge for about five days, but to enjoy them at their best and whitest, eat them within a day.
The method you use to cook a cauliflower determines how white it remains; for presentational purposes, the whiter it looks, the better. The whiteness stands out when served with a selection of other colored vegetables. Cook cauliflower quickly in a hot steamer or boiling water. Letting cauliflower overcook will sap the whiteness and turn it an unpleasant yellow color. Adding a few drops of milk or lemon juice helps preserve the color and can make it even whiter. Serve immediately once cooked.