My hairdresser uses semi-permanent dye that over-time fades with washing. The result is roots that aren't very different from the coloured area. It works well and as a result I only get the colour touched up once every 4-6 months (when I get my hair trimmed).
My hair is naturally black and I only get it dyed a few shades lighter (to a rich brown), so it might not work for drastic colour changes.
It probably isn't the best for me since I have psoriasis on my scalp but I get touch ups every 5 weeks or so.
My daughter used to get that, if you buy a product made in Australia by Moo Goo - for the scalp, it really does help.
My daughter tried everything but Moo Goo cleared it up in a week.
Moo Goo is available at many good chemists but also they are on-line.
Made here in Australia.
Just thought I would mention it as it is a great natural product.
http://moogoo.com.au/psoriasis/?SID=e9b3008ae40397704a41472669baede5
I can't go any longer than 5-6 weeks. 6 weeks is pushing it. I get permanent color to cover grey.