I am not familiar with grout bully but I have used grout dye to change the color dozens of times. Just do some research and find a good one. Dont use cheap priced ones as they are weak and ineffective. It may take 2 applications to cover.If the grout has previously been sealed then a dye cant be used as it wont penetrate the grout. The good thing is the dye is also a sealer if the grout was not previosly sealed. The dye can last for 15 years and will give a uniform color look .