Temporarily Color 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 a permanent with a 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.
However, you should avoid pulling that permanent root dye through to your ends every time you touch up. Doing this repeatedly can cause structural damage and color buildup, making your ends look progressively darker and duller over time.
Instead, a great solution is to use a demi-permanent gloss or toner to refresh your vibrant red ends. If the company that makes your permanent hair color offers a matching demi-permanent formula, you can apply the permanent dye strictly to your roots, and use the gentle, deposit-only demi-permanent formula on your ends during the last 10–15 minutes of processing. Demi-permanent color lacks the harsh ammonia of permanent dye and typically lasts about 20 to 28 shampoos, making it the perfect way to revive fading reds safely.

As for your wish for a mascara-type applicator: you are in luck! The beauty market now features a massive variety of temporary root touch-up products designed exactly for this purpose. You can easily find root precision mascara wands, targeted root sprays, and mineral brush-on powders at almost any drugstore or beauty supply shop.
These products come in highly nuanced shades, including excellent auburn and red tones. Because they are completely temporary and wash out with your next shampoo, there is zero risk of an unpleasant result or permanent damage. They are the perfect, mess-free solution to seamlessly hide your gray and brown roots and comfortably stretch your coloring appointments to the 6-week mark.
©Hairfinder.com
See also:
Hair color application
How to touch up gray roots
How to make hair color last longer
The different types of hair color