@Blondsabine,
Ash blonde, by definition, has more whitish highlights. I think adding anything reddish is going to look really off. Hell, I have medium brown hair (what hasn't gone grey) and golden blonde looks fine, but I've done more reddish and it tends to look jarring.
Do you have a photo you could post? It doesn't need to show your face, just your hair, if you're shy. Load it to a free photo sharing site like Photobucket or Image Shack and paste the IMG code in here (you can't just load it from your PC or phone). Anyway, a picture tells a thousand words, of course.