Lighten Gray Hair
A: You should simply be able to use a bleaching agent with developer (available from your local beauty supply shop) to lighten your hair. Since your gray hair is already without pigment, you can't really make it any lighter, so the only visible difference is going to be with the hair that is still darker.
Then there is the problem of dealing with potentially overstressing the hair in trying to lighten it. Even the gray hair with no pigment can be damaged by bleaching unless you use care. If the remaining pigmented hair is clustered in areas so that you have patches that are darker than others, you could use a foiling or cap highlighting technique to isolate the darker hair in order to lighten the overall look of the hair. If your pigmented and gray hair is evenly dispersed throughout the head, this isn't as viable a solution.
Seriously, my best advice is to speak with your salon stylist about the options he/she recommends for your hair if you only want to try and lighten the existing pigmented hair. Without being able to see your hair and know the exact pattern of gray vs. pigmented hair, I can't be sure how would be best for you to proceed.
That being said, and understanding that you have said you didn't really want to try to cover all the gray, you might consider using a high-lift color formula with a pale blonde color to lighten the pigmented hair and add a little color tone to the gray hair. You would have to do regular retouching of the color, but you would need to do the same if you simply lightened the darker pigmented hair as well.
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See also:
How does bleaching hair work?
How to highlight hair with a highlighting cap
Can I increase the progression of my gray hair?
Can I use a product to remove my hair color and go all gray at once?
