MY knowledge (limited 'cuz I wasn't at watercolor very long) Yorky posted 1 image for a monthly challenge.
Someone posted a
weekly sketch challenge. Generally one (somewhat simple) photo was posted along with several verbal ideas: a continuous line drawing of ___ (an ice cream cone) a memory drawing of (your day)
15 minute sketch of (a horse) , draw with your non-dominant hand something (from your kitchen) etc.
Of course, they were to be painted,
I don't know if gouache, being opaque, would work so well with these simple, quick drawings.
Paintings were submitted when done, everyone was encouraged to comment on the other works.