Do I still have to bleach it?

Well,my hair is medium brown with blonde and light brown highlights(I didn't get it done its literally natural)...and it changes color every winter and summer only...I want to dye it strawberry blonde like Rihanna did with her first angular hair cut.also in her videos "So Hard" and "Russian Roulette"...so do I still have to bleach it?and which would be better stripping it of color or bleaching?...