I've heard of "huevember"(involving promts+colors). Also "march of the robots"(drawing robots in march XD). I think I've heard somewhere "sketchember". But ppl tend to invent their own challenges so, maybe you can come up with one new.
I loved outcast, has a great pose and those eyes are pretty expressive. Fury reminds me of saint seya's armors and the poseidon saga, the red accents are on point with the prompt also! Adrift seems quite lonely and sad, and the blue just accentuates that feeling, quite cool!
RE: More may mermaids