Value, is King and Queen of the realm.
Get your values right and your painting will look great. Here is a bad example. An original photo, Samir's painting, and then my painting. See the issue?
See the difference in the grass? I sure do! I was scratching my head wondering what is wrong with it?
*Edit: I have fixed the painting. Here is the Monochrome result for comparison:
Still, a little bit too dark but livable now. I made a few more tweaks too.
Review your painting against the reference, in the mono setting on your iPhone and you will see your mistake(s). Then, when you notice the issue print the original reference photo; in color, then match your color and "value" directly on the printed photo. Test the paint color and value right on the paper. If the two match then you can fix your issues! I will post a revised painting here after I have fixed it. I have trouble seeing value versus color. I will have to work on this more.
Here is a good rule to follow:
Here is the updated painting. Mucho better!