I'm a stylist and there are different ways to do this.
It depends on your color and how much grey you already have though.
First, I would recommend to get either low-lights or hi-lights (depending on your coloring) through well placed foils to blend your colored hair color into your natural regrowth. As time goes by, the foils become less prominent until you can eventually stop as the color grows out.
I have also cut a woman's hair off at the gray line, she loved it!
this website has some great advice on greying and how to care for gray hair!:
http://www.virtualhaircare.com/virtual%20salon/what%27s%2...
"This really depends on the base of the hair colour when you make the decision to go "GREY GRACEFULLY".
We have had a lot of success by using the highlighting technique and turning it into a gradual process which can take a few visits and up to 6 months. This is best suited to hair that is light brown or lighter. What we initially do is a full head of highlights, bringing the hair to a very light pale blonde. We then tone with a "SILVER" based toner this will eliminate any yellow from the hair immediately. The idea here is to give the appearance of more grey hair than what is already there. This will make it a lot easier to grow out, as we lose that regrowth line pretty much straightaway'.
'On darker hair we have had success with low-lighting the regrowth area on the parting whilst allowing the rest of the hair to grow out. At a certain point the hair starts to look a little dull and here we start to introduce the aid of semi-permanant colours which we use as a camoflauge to the white hair. The advantage of this is that the colour fades out and allows the natural hair to grow through. Be careful though as, going too dark or being in unskilled hands can cause colour build-up and really defeat the purpose of going grey.
And finally the natural way. CUT YOUR HAIR SHORT and in 2 cuts your colour is gone. As drastic as this sounds, women who have grey hair always look sensational when they have a short striking haircut. There is nothing like this. Truly sensational'!"
Another option is to use a haircolor like Wella's Color Touch with a low volume developer. It has decent grey coverage in that it adds some color to your hair, but still allows some of the grey to show through. Basically, it creates a nice and natural blend.