Why are some animals and birds albino (without color)? And why albinos are few in number?

Sometimes an offspring can have a trait which is unlike the traits of both parents. The information about traits is in the form of DNA in genes, one of which may completely hide the effect of another gene and thereby become a dominant gene. A gene whose effect is eclipsed by a dominant one is known as a recessive gene.
Albino peacock
The rule of heredity mandates that whenever a dominant and a recessive gene pair together, the dominant gene controls the trait. In rare cases, however, a recessive gene overshadows the dominant one and causes an offspring to have traits that neither parent has. Every so often, the new trait results from a mutation, i.e. change in the DNA contained in gene. (DNA is just four basic chemicals or bases–A, C, T and G–strung together like steps on a spiral staircase). The change occurs when DNA makes copy of itself, wherein the whole or small section of the DNA is rearranged. As a result, a different DNA message is passed along to the offspring.
One example of mutation is albinism, a condition in which a bird, animal or a human being is unable to make a substance called melanin. It is this pigment which in different concentrations gives coloration to the eyes, skin, hair, and feathers. Lacking melanin, albino organisms are usually colorless and appear snow-white. In the wild, albino birds and animals can not stay as safe as other members of their species. Normal birds and animals, using a combination of various colors, provide themselves which elaborate and highly effective disguise. They have evolved an uncanny likeness to elements of their environment such as leaves, rocks, grass and pebbles to avoid being noticed by predators. Albino types are easily noticed, and so cannot hold out for long. On the other hand, an albino predator (such as snake) is easily noticed by its prey, which makes a hasty and timely retreat. A snow-white peacock, shown above, has its own problem. It cannot attract a female, due to absence of the bright purple, blue and green feathers. This explains why albinos are few in number. In evolution, their condition stands to their disadvantage in many ways.

Additional reading:
Albinism (Wikipedia)

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