The colored lines on the cats are nice.
It's good that you added a variety of colors to the background.
The cats are well drawn considering you were using a mouse.
You didn't use Yellowfang's tail to cover up her paws, which shows you weren't being lazy with the picture, which is always a good thing.
The orange sky is nicer than just a black/ blue sky. it could use hints of red and/ or pink to give it more color, but overall it's nice.
Yellowfang has yellow teeth, to start off. The teeth for her in this picture look white.
Also, Brokentail's markings are darker .I would suppose that would make his facial features harder to see, but just a little darker wouldn't hurt. Also concerning Brokentail, his exposed skin shouldn't be as shiny. Two different shades of pink would be nice. And the skin doesn't need to be as bright, try making exposed skin a little more pale in the future. To add on, you forgot to shade him. That happens, to a lot of artists, so you shouldn't really worry about, much. Maybe you could edit the picture to fix it.
Even though it's nice that you somewhat shaded the background instead of leaving it blank, It just looks like a bunch of scribbles. Try adding more detail to the background. It's just as important as the characters in a picture. For example, have vertical lines going in the direction of the grass or bumpy horizontal lines to show the texture on the dirt.
Something that doesn't matter as much but I would like to bring up is that Fireheat should be watching from the cave entrance.
Remember, the only reason the "Cons" section is so large is because I'm explaining in detail what you need to fix, not necessarily that there are a lot of things wrong with the picture. You actually did a really nice job in creating a good picture. ^^