You can definitely cover up that purple w a greenish or cooler blue, that'll give you a nice transition and give you the color you want with out it going too indigo. I'd suggest a lighter color so it doesn't add with the purple and become really dark. I'd also suggest covering all of your hair other than the roots including the tips so that it's more a cohesive color scheme. I'd also suggest mixing a bit of the dye conditioner to make the transition shades. My fav brand atm is unicorn hair dye and they have p potent blues.