It's a lot of mixing and having to figure out the mixes, but it hasn't messed mine up yet. My hair also grows like crazy though so maybe I just have the weird hair xD
Most likely not but I personally wouldn't dye my entire hair since depending on the hair it can damage the roots and the hair itself really bad I've seen more than enough cases like that
Granny Dyeing hair to often can ruin it entirely you friend is the perfect example for that I'm still considering dyeing my hair tips in a dark purple tho lol
I have a friend, who colors her hair like every 6 weeks. Over time, it damaged her hair roots so bad, her hair stopped growing and now she has to wear wigs.