The color challenge thing isn't so much of a contest but rather a general theme that people can participate in if they wish. Different colours for different days; it's just for fun, so there aren't many rules as to what would qualify, it sounds like the grass would be green in the picture you described so...sure, I think it would be fine.
I did stare at the butterfly for quite some time trying to get a good shot of it!
RE: Butterflies and Friends