Hair Coloring Neophyte Questions

I just colored my hair for the first time and I really love the depth and shine that my hair`s never had before. I`m already looking forward to the next time, as I try to alter the shade a bit to reach the perfect one (the impossible dream). I went from dull light-medium brown to "chestnut" (L`Oreal Castings) which is a med-dark brown with red highlights. My next iteration will aim for a little lighter and maybe a little more red. I`m happy to take suggestions, but that`s not the goal of the message. I have a couple questions. First, when I do it again in roughly a month, will I have to consider the current color? I know that semi-permanent color can`t lighten, but since I plan to go darker than natural, my inclination is that I should be OK. Second, given that I`m not going lighter and not covering grays, is there any advantage to using permanent haircolors? Thanks in advance.