A quart typically covers ~ 100 sqft. A pint or 8oz per color ought to do it. Pints at a big box store are less than $5, and one seems to have a 50% sale.
Google image search for mountain wall mural layered to find other's works. Try to identify what it is about the ones you like that works so well. Also, take into account your flooring color, furniture colors etc.
Print out a color image of one of her large photos (maybe take the URL to Kinkos or Staples or photo store and ask them to print it to 'match'), take the print to the store, find color swatches that match.
Here is a blog post that describes the tinting:
http://www.trailtosummit.com/create-a-mountain-mural/
Here are very detailed step instructions, including a photo showing all the paint chips and their codes:
http://kristinatravels.blogspot.com/2016/06/how-to-paint-mountain-mural.html
Your person wrote a blog post about her process:
http://pamlostracco.blogspot.com/2014/05/mountain-mural.html
Here are time lapse videos showing a different people doing similar:
https://www.youtube.com/watch?v=iQGZi75w5LY