This was a hard one for me. When I think of monotone a sound in one pitch or somebody speaking without emotions. That was hard to photograph, but then I got to dull / one color / boring and I straight away thought about the sky. In Oman we hardly have clouds.
When my parents visited in January, they were very excited to open the curtains each morning and see the blue skies, but after a week they started to understand that it gets quite boring after sometime.
Anyway, here is a picture of the afternoon sky. To try to make the picture a little more interesting, the palm and wood-apple tree are there too.