I am a hairdresser and taking a poll if you will ...

I know that the color doesn't stay in very well.  Even if color says it's permanent, it fades, and sometimes doesn't work on certain pieces of hair.  You an go to a salon and ask them to dye the pieces that were missed. 

i have been a hairdresser for a number of years and this is a very common question.  in my experience the grey hair is resistant to colour and there for it doesnt last as long but you could ask your hairdresser to change the peroixe strength.

I have always heard that gray hair is more resistant to color.  I was born a dark haired brunette and had kept my hair darker with hair coloring. Finally in my mid-fifties I moved back to Southern California from the Northwest and decided to have my hair lightened so the roots wouldn't be so obvious when they came in.  It worked and I finally got to see what life would be like as a California blond.  However, knowing that warm tones are not the best for my cool winter complexion I recently decided to "go off the bottle" and go natural.  (I loved having light hair surrounding my face but knew I didn't want to go back to coloring my hair a dark color).  So NOW I have naturally light hair without the time and cost of anything other than a stylish hair cut and am happy to see that so many other babyboomers are doing the same.