If i dye my hair a dark brown over medium/dark brown hair with red dye in it should i swim?

I also have blonde highlights and i am going to the salon tomorrow for a cut and i want a color but my grandma is making me wait until summer is over before i can get it dyed because i will swim sometimes. She says the pool will wash out the brown hair dye and the red will come back if i swim in the pool. Is she right and if not then how to convince her to let me get my hair dyed dark brown before the end of summer? How long should i wait before swimming if i went dark brown -black. My natural color is medium brown.