I found another interesting article

"Other than green, purple is the most common color in living marijuana. Triggered by seasonal temperature changes (just as trees change color in the fall), the vibrant hues of the mature pot plant are also the result of genetics and the creativity of the cultivator or breeder.

Indoors and outdoors, color can be influenced by deficiencies as well as by temperature. The intense green of chlorophyll usually overpowers other colors, such as red, orange and the coveted purple. Chlorophyll tends to break down late in the season, and a pigment known as anthocyanin is unmasked and allowed to show through.

For purple color to develop upon maturation, a strain must have the genetic potential to produce anthocyanin pigments. However, the color might never be shown if environmental conditions donā??t cause chlorophyll breakdown. Colombian and Hindu Kush strains tend to develop purple coloration when subjected to low night temperatures during the end of their life cycles. Purple Orangutan, originally from Afghanistan with heavy traits of the Hindu Kush, produces some of the darkest purple and blue tones found in any marijuana strain.

Another pigment called carotenoid is largely responsible for the yellow, orange, red and brown colors of marijuana. These colors begin to show in the leaves and calyxes of certain strains as the green chlorophyll color fades. Gold strains reveal underlying yellow and orange pigments as they mature. Red strains are usually closer to reddish brown in color, though certain carotenoid and anthocyanin pigments are nearly red, and streaks of these colors occasionally appear in the petioles (the slender stems supporting the leaflets) in ripe flowers. The red color in pressed, low-grade tops often consists of masses of reddish-brown dried pistils."