Definitely do not bleach it if you plan on having it wash it. I use semi-permanent hair dyes (Manic Panic and Punky Colors) really often. They've never completely washed out.. but they were easy to fade if the hair wasn't bleached first. It's actually kinda weird.. I find that semi-permanent dyes last a really long time.. much longer than "permanent" hair dyes. When I dyed my hair blue, it lasted for months and months and barely faded.