How to paint the wall with gradient patterns?
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 store seems to have a 50% sale on these sizes.

To find works by other people, Google image search for: mountain wall mural layered. In comparing them, try to identify particular aspects of the ones you like that make them work so well.  Also, relative to those murals, take into account your flooring color, furniture colors etc.  This will aid in assessing whether to 'tint with white' or get the store to mix each tint.

Print out a color image of mural of your choice (maybe take the URLs to Kinkos or Staples or a photo store and ask them to print it to 'match'), take the print to the paint 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 wine and including a photo showing all their paint chips with 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

https://www.youtube.com/watch?v=QPE5G2SR0Zs

https://www.youtube.com/watch?v=QVQ8mPmSZS8