This is not written by me! It is copied from another 'private' forum from a user named Marlon Machado
The glaucous green or blue colour of many species of cacti is due to the deposition of a layer of wax crystals on top of the outermost layer of the "skin" (epidermis) of the plant, the cuticle. This layer of wax (called epicuticular wax) is deposited on top of the cuticle cells shortly after these cells are formed at the apex of the cactus stem. Because this layer of wax is produced only on young cells and is not replaced later, as times passes this layer of wax is slowly removed in plants exposed to the elements - that is the reason why young stem growth is always more glaucous and greyish or bluish than older parts of the stem. This layer of epicuticular wax has several functions: it serves to reinforce the waterproof quality of the cuticle so that the plant does not loses water through the epidermis; it functions in defense, forming a physical barrier that resists penetration by virus, bacteria and other disease organisms, such as the spores or growing filaments of fungi; and lastly it serves to reflect part of the excess solar radiation (sunlight) that the plant receives. Perhaps this last property is the most important to explain the blue colour of many Pilosocereus species. But before we can discuss that, first we need to remember the relationship between plants and light. The sun emits a range of different radiations, and light is just but one of the many kinds of radiation which are emitted by the sun. The radiations are emited by the sun in the form of pulses or waves, and the distance between the crest of two consecutive waves, termed a wavelength, is how we identify the different kinds of radiation. Wavelengths varies in length from nanometers (one billionth of a meter) to several meters. The wavelength indicates how fast are the pulses of a given type of radiation, and thus how much energy it carries: short wavelengths are more energetic, while long wavelengths carries less energy. The difference has to do with the amount of pulses in which a given radiation is emitted in a given unit of time: a radiation with a short wavelength will have many pulses emitted during a given time, while a radiation with a long wavelength will have less pulses emitted in the same period of time. Because of this, more energy per unit of time is transmitted by radiations with short wavelengths than radiations with long wavelengths. The sum of all kinds of radiation emitted by the sun is called the solar spectrum, and light is nothing but the spectral range of solar radiation which is visible to the human eye, and which ranges from about 380 nanometers to 780 nanometers. For humans, this range of radiations is translated in colours as follow: range of radiation between 380?450 nm: violet range of radiation between 450?495 nm: blue range of radiation between 495?570 nm: green range of radiation between 570?590 nm: yellow range of radiation between 590?620 nm: orange range of radiation between 620?750 nm: red Plants absorb light radiation though a special pigment, the chlorophyll, and convert this light energy into chemical energy through a process called photosynthesis - which is basically the process in which plants manufacture their own food using the light of the sun, water and carbon dioxide. Thus, plants need to absorb light in order to live. However, plants do not absorb visible light through the whole of its range, but there are two peaks where absortion is at a maximum: between about 400 to 500 nm which roughly corresponds to the blue colour of the spectrum, and between 600 to 700 nm which roughly corresponds to the red colour of the spectrum. The range that is not absorbed, from about 500 to 600, correspond to the green colour of the spectrum; because this range which is not absorved by the plants is reflected, this is the reason why plants appear green to us. The range of radiations which are useful for the plants to perform photosynthesis are termed Photosynthetically Active Radiation, often abbreviated PAR. Now lets go back to the third function of the epicuticular wax in cacti, which is to reflect part of the excess solar radiation (sunlight) that the plant receives. The green cells in plants can process a limited amount of light through photosynthesis at any given time, and the excess light radiation can be damaging to cells and tissues, specially the more energetic radiations of shorter wavelengths. The same thing occurs in humans regarding ultraviolet (UV) radiation, which is the sun radiation of wavelengths below about 400 nm. Moderate exposure to UV radiation is healthy because it estimulates the skin to produce Vitamin D3, a vitamin whicht plays an important role in the maintenance of many organ systems in our bodies; on the other hand, prolonged exposure to UV radiation can cause skin cancer. Plants that live in areas with high intensity of sunlight cannot absorb all the light that they receive, and need to devise ways to prevent damage to their tissues by the excess sunlight. Normal leafy plants usually have leaves that live for a short period of time only; if a leave is damaged by excess radiation, the plant drops this leave and simply grows a new one, thus basically avoiding the problem. Xerophitic plants that live in dry areas lose their leaves periodically during periods of drought. Cacti however do not have this luxury: their epidermis have to live and perform photosynthesis for many years. Thus, in order to protect their epidermis from damage due to excess sunlight, the cacti evolved different mechanisms: some decided for a partial shade strategy, and developed a high number of ribs which partially shade each other, or a dense cover of spines which partially shade the stem, or long hairs like those of Espostoa or Oreocereus which perform the same shading function. Other cacti and many succulent plants with long-lived leaves, like for example Echeveria and Dudleya of the Crassulaceae, evolved to produce a very thick layer of epicuticular wax in their epidermis, which reflects the excess solar radiation. This layer of wax or farina as it is sometimes called usually confers a greyish to bluish colour to the epidermis of the plants, and the reason for the bluish colour in particular is because blue, violet and ultraviolet are in the short wavelength range of the spectrum and thus more energetic and therefore potentially more harmful to the plants. In short, the advantage of the glaucous blue colour of many of the Pilosocereus species is to reflect excess solar radiation that would otherwise be harmful to the plant, and the reason for the color blue in special is because this colour is composed of radiations of shorter wavelengths, more energetic and potentially more harmful to the plants. Fruits of Pilosocereus and other columnar cactus species are glaucous green or bluish when they are unripe, and the reason for the colour is also a dense cover of epicuticular wax. Usually this protective wax cover is lost and the colour of the fruits change when they are ripe.
http://webcache.googleusercontent.com/search?q=cache:d2hZYd-btp0J:www.bcss.org.uk/foruma/viewtopic.php%3Ff%3D41%26t%3D74233%26start%3D30+&cd=17&hl=en&ct=clnk&gl=au The fact the Pilosocereus need more warmth during the winter does not have anything to do with their color - these plants grows in warm areas in the Americas, and are not used to grow in low temperatures.