How often can I color my hair, and what do I do about my roots in the meantime?

I color my hair a dark auburn-red color that is quite attractive when the color is fresh and for a couple of weeks thereafter. When my brown and gray roots start becoming noticeable, however, it doesn't look quite as hot. Moreover, because red hair color tends to fade more noticeably than other hair colors, I almost always have to freshen up my overall color (by spreading the dye around to the ends for the last 10 minutes) when I touch-up my roots.

I wish I could color every 3-4 weeks, but I'm afraid to color my hair more often than every 5-6 weeks. Is coloring hair more frequently than that damaging? How often can you safely color your hair?

What can you do about roots in the meantime? Can you rotate permanent with non-permanent dye? Are there any good products you can use to temporarily color roots? (I apply a tinted toner called "Stay Red" to visible roots after washing my hair, but it's hard to apply. I wish they had a mascara-type applicator.)

Thanks for the tips!