techincally its chemistry, but I prefered the aliteration of this subject, so anyways: I wake up yesterday and out of the blue, decide my hair could use some excitement. A couple toxic chemicals and a blowdry later, voila, blue hair.
Now I’ve only had my hair dyed a couple of times, but I do recall that previously, the dye used was permenent. I enquired about my options for blue coloring and was informed that a permenent blue hair dye has yet to be invented. Now while most of you dopers are probably busy pondering what on earth would posess someone to even temporarilly (2 weeks) dye their hair blue, I am curious why its molecular structure of follicles permits permenent red dye for instance, but not blue?
cheers…and any advice on how I’m going to rationalize this to my 'folks would be most appreciated too 
Permanent red dye? Where? I’ve tried to get red hair for years, and it all fades away really quickly!
The thing is, the dye isn’t really permanent - it just lasts longer than non-permanent dye. My friend dyed her hair blue once - it was purple within a day or two, and then faded to an odd grey. just to give you a head’s up!
Your hair is a bunch of dead cells, being held together by a bunch of proteins which are just really stable together. The hair cells slough off, and your hair grows out, and the proteins degrade, and the ones that were bound to dye molecules essentially “fall off” when you wash your hair, or comb it, or just by walking around.
The reason some dyes work longer than others probably has to do with the stucture of the dye molecule itself, and its binding to your hair. If it doesn’t bind all that well, the dye will fade a lot sooner than one that has a very strong bond. Also, there are very likely some degredation effects happening just from the UV-light from the sun. A lot of compound degrade when exposed to UV light.
Reds tend to be more “common” natural colours, and so a red dye molecule is more likely to be stable than an unnatural blue molecule. Besides, a lot of organic chemicals (which I assume hair dye is - I’d be surprised otherwise) absorb strongly in the UV and near-UV ranges, and so are more likely to appear yellow or red to us. Red-absorbing (blue molecules) are much rarer without adding something like a metal ion which might appear blue (Copper 2+, I think - or 3+ I never remember). So the synthesis of a stable, high-binding affinity-red absorbing-blue appearing molecule that is safe for your hair is a lot more difficult than extracting an existing red dye from a plant.
Just so you know, all of this is a WAG, but I am an organic chemist (in training) and so some of it may have some basis in fact. Or not. 
I have blue hair! (Actually, only 1 streak down the left side of my face is blue. The rest is dark brown.)
How to keep it nice and colorful for more than a couple days–don’t wash it. Seriously, just rinse it, your hair will be fine. I wash the rest of my hair, but not the blue part.