Since your bathroom is small and your walls/floors are gray, in your new shower you could consider a white subway tile with a border of glass mosaic tile that includes some gray and white that is placed at a higher level in the shower so as to draw the eye upward to the vertical space in the bathroom given its small footprint.

There are acrylic pre-formed shower pans that you can install and then just tile up your walls. It's a lot simpler than building up your own shower pan. There are also pre-formed kits that include the shower pan and acrylic walls to replace exactly what you have. Hopefully there's cement board and waterproofing behind your existing shower. If not, you'll have to add both to ensure your shower is completely waterproof. Good luck with your project!