Genetics and the soil it is planted in (from all accounts). According to the below link, there are some you cannot change and some you can with some degree of success. I would get a cutting from your friend's neighbor for best results.

http://www.hydrangeashydrangeas.com/colorchange.html

Thanks you hc mcdole. So I guess my confusion is in that I was told that hydrangeas only come in various shades of blue and pink besides the whites/greens so no purple. Is this right?

That's correct. The best shot at purples includes two options: a breed that does either deep/vivid pink/blue in somewhat acidic soil - or - a breed that leans towards pink in highly acidic soil.

Thank you for clearing that up!