Q) I'm tattooïng for five years now. When i put in color, i always have to give it a second pass after three weeks, when the tattoo is healed, because it's never bright and smooth enough. Doesn't matter if i work fast or slow. I was wondering if i'm doing something wrong?
A) Most likely it's not the pigments- these days most tattoo pigments are capable of looking bright and rich in the skin if applied correctly. So your saturation is going to be affected by either your machines or your technique. You need a good machine with a long stroke, opposite of a liner. it should have minimal spring tension- not much more than needed to keep the front spring in contact with the contact point when the machine is at rest. A gap of 1/8" or more is about right. This gives you a slower stroke that allows you the right kind of overlapping hand movements that allow for good color saturation. I also recommend spread magnum needles, which are a good point arrangement for making overlapping strokes without the multiple passes causing trauma.
As always, make sure you have a consistent tight stretch, and take your time doing those overlapping strokes until you have the rich saturation you want, keeping in mind the skin's natural limits. If you find it to take too long, try a larger magnum- the mag size should be proportional to the size of the tattoo.
When you're done with the magnum, you can improve saturation by doing a tightening pass with a small round group... use it to tighten color areas against their outlines, pull it through tight spaces or saturate areas more that you didn't hit evenly enough the first time around. By doing a tightening pass, you not only give the piece a sharper look but also add more pigment to the piece, improving the saturation. Most of the tightening work is done in areas the magnum couldn't really saturate very well, meaning that you can do this without undue risk of overworking the skin.
Even with stellar technique, a second pass will allow you to apply more pigment than the skin could naturally handle in one pass. That's a fact of life- two passes is better than one. For my large custom work, I usually try to get the client to come back for a follow-up pass, but not after three weeks- that's barely enough time for the skin to heal, and not nearly enough for it to settle. 6 months or more is probably better, if your client is committed enough to the process.
Last but not least, your use of shading and color- from a design sense as opposed to an application sense- will contribute to giving the piece a high-contrast saturated look. Having a good sense of color theory can more than make up for some weakness in application. Draw and paint as much as you can to really experiment with color. You can also read more about color theory in Reinventing The Tattoo, or in Nick Baxter's awesome painting instructional book, Tight-Focus Realism In Oil.