If you're up for changing the shower tile, I say do it! Choose a simple inexpensive almond or light natural color to blend with the floors, trim, and countertop. And have it laid out in a subway pattern to bridge between classic and modern style. Then look at creamy neutral paint colors like 7512 Pavillion Beige, 7527 Nantucket Dune, or 6148 Wool Skein. Or you could go to a pale green with the towels to complete a fresh and light makeover.
The change will definitely be very dramatic from dark green to light or neutral. But when it comes to selling a house, most people are afraid of color. They also immediately try to picture their own things in the new space and your colors could limit their imagination. Your home doesn't suddenly have to be completely bland to sell, but it does need to have some broad appeal to get the most offers and best price.
You're on the right track. You've got this :-)