Have you considered a natural hemp dye?

AR has a good suggestion, a hemp dye. There's also henna dye, which is natural and doesn't damage.

For more commercial dyes, I like Clairol's Natural Instincts line. I find it makes my hair feel healthier, actually. It's temporary, lasts up to 28 washes. I wash my hair once or twice a week, so that's like, 4 months!

How dark do you want to go? Because some hemp/henna dyes can stain darker than you want them to.

I was going to mention the Clairol Natural Instincts too. That's always been really gentle, and eventually just fades on its own so you don't have to worry about growing your hair out. (Important for those of us with really long hair.)

If you want something that's not going to be too harsh on the hair go to Sally Beauty Supply and look into their demi-permanent or semi-permanent dyes. Not as harsh as permanent or the other stuff you get right out of the box.

I bleach the hell out of my hair so I need gentle stuff, but I always go with N'Rage or Manic Panic's crazy colors. They have semi-permanent in natural colors, too.

I used to work at a Sally's and did a lot of playing around with my hair. It's your best bet and they can tell you what would be best for you.

I go to the salon every 3 or 4 months and let my hair guy deal with it. But my hair is pretty gray and I have to do maintance at home. I use Loving Care just at the roots about every 4th or 5th shampoo. It is very gentle! If you are going darker this might work because you could start off using color all over and then just touchup as new growth appears. I find doing this refreshes the color everywhere.

If you already are dark, I'd use amla or buxus or any indian herb that's made for haircare. It'll darken them gradually, naturally and without hurting your hair.

I don't really know about them. I rarely dye my hair. This was an interesting post thanks for posting it.

I've never dyed my hair until about 5 months ago. I had virgin hair. I was afraid that I would damage my hair. I decided to go with an intense red shade. I've heard great things about Garnier Fructis dyes so I decided to try that. The dye smells delicious once all the ingredients are mixed together. I dyed my hair about 4 times in a 2 month span of time to get the hair color I wanted because I have dark brown. The box comes with dye along with a conditioner for use after dying your hair and is AMAZING!2 It smells great and keeps your hair really moisturized. 4 months later and my hair is softer than ever! The dye didn't damage my hair from what I can see. I love this brand of dye. I'm planning on dying my hair blue black soon.