Three colour landscapes

Several years ago, my local art group set the evening's two hour workshop members the task of producing a landscape with just three colours.

Instead of going for just one image, I decided to try the idea out on a very small scale (about 3.5" x 2.5") and produced four experimental paintings.  The subjects were imaginary and were only intended as exploratory colour sketches - an interesting exercise.  I am not sure if I used the same three colours for all the sketches (I can only find one original in my old files, but my notes indicate that I used Rose Madder Genuine, Cobalt Orange and Cobalt Blue on Saunders cold pressed 300lb watercolour paper - the first image shown below).