Well lets just start off by saying you did a fantastic job on this! The value contrast you've used brings a lot of attention to your subjects and makes for a very interesting piece.
The colors you've used look really good together. The greens are drawing a lot of attention because of the hue contrast, but that actually works really well compositionally: people like to follow the gazes of your characters. In this case it's doing a very good job of leading your eye around the picture.
Lets see if I can find some things that can be improved...
Well the first thing that comes to mind is the hair. Opal's hair and Sweetie's hair look like they were drawn differently. I'd say Opal's hair has the right amount of detail here, but the texture seems a bit rough. Here's a little edit I did with the smudge tool to kind of show you what I mean. [link] Sweetie's hair needs more attention though, her tail in particular looks a bit flat.
The hard lighting from below looks great, but I feel like the broom should be casting more of a hard shadow in some areas. Mostly on Opal's head.
The highlights on Sweetie's eyes could be a bit brighter and harder. Eyeballs are just about the smoothest surface you can find. So highlights tend to be very hard, almost like a mirror. Also they should probably be at least as bright as her coat, as eyes are more reflective than fur. Cosmetically it goes a long way in bringing your subjects to life.
Over all I'd have to say that I love the colors and I love the lighting. The anatomy looks good, the composition is solid as far as I can tell. This is going straight to my favorites.