What is the best way to recolor 1970 green bathtub and wall tub tile?

Ceramic tile can be refinished/re-colored... there are companies that will come to your home and do it.... (they have all the equipment to do all the prep and finish) much cheaper that replacing the whole thing... and if you have it professionally done it will last a lot longer than just painting it yourself.

Try Home Advisor, or Angie's List, or search "tile refinishing" in your area... make sure you get references.

Paint it! There are a number of products on the market that you can pick up at any big box home improvement store.

Rust-Oleum, Homax Tub and Tile Paint Kits.

Unless you totally and utterly hate it let it be those colors are coming back I would paint walls a deep deep eggplant color or almost black. That would make color pop. That was a very important decade and it sure would be a shame to lose it for bland colors of today. I have a very traditional 1984 home and it's cold or better yet has no character, those 80's had bad music and home styles. So I keep adding the different decades that I like and bold colors nothing bland boring and cookie cutter within the box stuff. I don't think that paint tile and a bath tub will last unless this is an unused guest bath. Humidity will take its toll. Menard as well as other big box stores are carrying a product that is a surround and it's not hard to do since you would just cut out the tiles and replace it with this item it looks like tile feels like tile but hey no grout to scrub so you won't lose any paint. You could pick a nice deep grey and keep that tub or go with white tile look and they have natural stone feel and looking tile as well. It won't go over the existing tile just attaches to studs go online and you can see it. But unless it's broken save your money because new homes regardless of a new build or a 70's Era will come with costly surprises your home inspector can't always see nor find. Just a thought.

http://www.hometalk.com/diy/bathroom/showers-tubs/how-to-paint-a-bathtub-easily-inexpensively-30576162