Right now your colors ramp is very linear : for the brown wall for example, you progressively darken the shade of your main color.
It doesn't look bad, but you can make it more interesting to the eye by using hue-shifting (look up some tutorials, hue-shifting is basically when you change the hue (color value) of your main color towards another color).
I shifted the darker shades of your first wall towards purple, and those of your third wall towards green, so it looks less monochromatic.
(Sorry about the crappy explanation, tell me if you didn't understand anything I wrote haha)