If you have wanted to put something other than paint on your walls, artwork prints could be alternative though they may be expensive. I’ll tell you that they are much cheaper than masters’ originals.
But if you want something original, why not take your digital camera out and make your own ‘arty’ pictures. Try to find colors around you in the forests, cities, and countryside wherever you go. You can get your photos (if they are good enough resolution) enlarged to poster size by any number of online/offline print bureaus.
So get rid of these plain walls. Put a little color on them, too!