Permanent over a demi-permanent?

Does anyone know if there are any consequences for applying a permanent color over a demi? I used Natural Instincts to go from medium/light brown to red and really liked the results, but the color washed out too fast and I miss the red hair! I don't think it's completely gone though, my hair is still slightly lighter than it normally is and if I look carefully I can see shades of red clinging to the hair. How will it affect my hair if I put a permanent red over a (very slightly) unnatural base color? Should I wait another week or two for it to completely wash out or will I be okay? Thanks!