Yes, you need to bleach first!
I have pink, magenta and purple in my hair. (Special Effects Atomic Pink, Virgin Rose and Wildflower.) I have my stylist do the bleach and inital color application and then do my own touch ups. You're right about the roots - I don't mind the roots at all because who really has pink hair? LOL!
I use Special Effects - it's way better than MP. I love their Atomic Pink. Atomic Pink doesn't fade out, it grows out. I clarify before touching up, dry my hair, then apply the color and cap it and hang out with it for a few hours on my head. Do vaseline around the ears and hairline or it will stain your skin! I only need to touch up every 3 months or so, the color stays vibrant and fun for a very long time with Special Effects. When I was using Manic Panic, I had to touch up every week to keep my color vibrant and bright. I touched up overnight, with a clarifying wash first then I dried my hair, then applied color and capped, and carefully wrapped it in a towel I didn't mind staining and slept like that, then washed it out in the morning. I buy SE from punk.com, they recently had a bogo 50% off sale and it might still be going on. I highly recommend them!
Don't use shampoo to wash it after you color. A little cowash is fine, then a nice deep conditioner to restore moisture. Heavy protein pulls out my color and my curls, but YMMV.
Happy coloring to you!