One of the biggest problems with permanent pigments is that the
best ones are unsafe for use on humans. This leaves the the
industry with just a few save alternatives. For the most part, very
light fast and permanent colors are used. But this is not
always the case. When it comes to yellow, purple, orange or
magenta, the fade resistance is still not very high. The most
common reason for fading colors is Ultraviolet Light,
especially the light from the sun.

This does not mean once you get a tattoo you have to stay out of the
sun forever. It’s just that excessive amounts of sun will fade
tattoos. It fades about the same rate your skin degrades when
exposing it to the sunlight too much.

Another factor is the application of the tattoo itself. I have noticed
the more experienced an artist is the better he can apply the pigment
correctly and the less it fades.

People often say their tattoo looks much older than it really
is. All I can say to it is they probably exposed themselves too
much to the sun, didn’t take care of it properly during the
healing process or the artist simply did not apply the pigment
correctly.

Your skin ages and with it the tattoo. That’s a fact and there
is no way around it. If you want the tattoo to look good as long as
possible take care of your skin and you will take care of your
tattoo at the same time.

source: http://tattoojoy.com/tattoo_articles/why_do_colors_fade.htm