i agree with the general comments, it does look a little flat. as a general rule, the further you go, the more blue in your mix. also, reduce the constrast on far away object. distant hills are generally just a little shade of greyish blue, compared to the sky.
dont be afraid to use artistic licence to "improve" on your photograph. even if the hills are very green on you photograph, just dull them out. it will bring the foreground out.
furthermore... i hope you didnt use green straigth from the pan, or tube.. it's a big no no (unless you are very experienced) !
always mix your green from a blue and yellow, preferably the ones you used everywhere else in your painting.
hope i am being constructive without being patronising !
keep it up !