Drab another day?

Hi there!

Would any of you vets care to answer a quick question? I'd really appreciate it. :)

I am a natural ash blonde whose hair has darkened over the years (I'm 30) from a medium to dark tone. Back in the day, I pretty much tried every color out there at least a few times, but rarely colored my hair over the last seven years -- perhaps every once every other year or so. I used to be quite good at it, but since my hair has darkened, the effect of hair color has also changed.

When I added some store-bought, cool-tone highlights to my hair last month, they brought out considerable red tones -- a sort of strawberry blonde.

I ended up buying my natural hair color in the Nice n' Easy line, adding drabber, and that corrected the tone, with extremely natural looking highlights. However, with minor sun exposure and shampooing, some of those same highlights have again turned a little strawberry blonde.

Will red tones appear each time I highlight my hair? Is there anything I can do to prevent it? Should I be adding drabber everytime I color, even if it's straight highlighting?

And is there anything I can do to minimize the red tones as the summer goes on, besides redying it with drabber?

Thank you for any help any of you can give!