Grey hair is less porous, thus won't grab and hold any color for any length of time.

I've gone a few shades lighter with some much lighter hilights and I can't see the grey at all. But my hair grows really fast and I have roots showing around 3 weeks so I see my stylist every 5 weeks.

A good shampoo that won't strip the color will help extend your color. There are many out there made specifically for colored red, brown, blonde, etc. hair. Condition the heck out of it too so it doesn't get dryed out.

I found my first greys at 17 and have been coloring ever since. I did have the good fortune of working in a hair salon many years ago and learned some good tricks as well as all the fabulous things they can do with color.