How do I get my highlights to hold a new color?

A couple of months ago, I went to a salon and got my hair highlighted with purple and LOVED it... for about a month. The problem is it washed out extremely fast. Now, I have started dyeing my hair at home (let's face it, the salon can really hurt your checkbook), unfortunately, even though I am using Feria Midnight Black, which puts very subtle blueish highlights in my hair, it also washes out extremely fast leaving me with this strange grayish color. I am only 28 years old and certainly am not wanting to have gray in my hair quite yet! Does anyone have any tips about what I can do to make those highlights hold my color longer? I would like to not have to spend a fortune at a professional and am pretty adept at doing my own hair. Any tips would be great!! Thanks!