I recently made a flier to give some tips for the maintenance of fashion colors in the hair. This includes all rainbow colors, neon colors, pastels, and other "artificial" tones. I am going to list these tips here and why they are helpful.
First, wash with cold water. This will keep the cuticle closed and help keep the pigment adhered to the hair shaft.
Second, use color safe shampoo and focus most of the washing on the scalp. The less friction the hair is subjected to, the longer the color stays.
Third, wash less frequently. Washing about twice a week should keep the color fresh for longer. For the same reason as listed in the third step.
Fourth, use a dry shampoo in between washes. This will help absorb the natural oil from the scalp and prevent the hair from seeming greasy
Fifth, avoid heat. Heat breaks down the color molecule and encourages fading.
Sixth, if you must use heated irons, keep the temperature below 325* F and use a heat protectant. This will help protect the hair and avoid excess fading due to the heat.
Seventh, avoid chlorinated swimming pools. The chlorine in the water will pull the color right out of the hair.
Eighth, maintenance of fashion colors is about 4 to 6 weeks. Even with the best of care, the color will fade and become more pastel toned.
I hope this covers the basics of owning a head of brightly colored hair. These tips are the best I have to maintain the fashion colors that are popular.