if you want the pink to show up well, yes, you have to bleach it first. from my experience as a colorist, semi-permanent bright colors do not show up nearly as well as permanent. depending on how dark your color is right now, that plays a part too. if you are a natural light brown, with no color on your hair at all currently,  you should be just fine with the semi.  other than that, id go the bleaching and then coloring route. hope i helped! :)