Besides the comments already posted about hair and fists, I think I'd try something between an olive drab and a goose-turd green for the dress and a colder, darker red for the background, like iron only just starting to get hot. The bright green is the wrong kind of discord, but you do want the colors to sit uneasily next to each other and look distasteful. Likewise his clothes, maybe slate gray or low-saturation vert de terre? He blends too much; I'd want him dull and in a 'receding' color but still visible enough to throw something at. Folding his arms sort of expresses him being unreceptive, but I'd tilt his head back and a bit to one side and shift his weight onto one foot to make him look more uncomfortable and frustrated. He's too static / passive here to have interested her enough to be that incandescent. I think I'd narrow her eyes a bit and focus them on the horizon instead of the viewer, or through the viewer, to match the rest of her face. On the other hand, along with a couple of the previous posts, what is the target audience and how would this move them to pick up the book? Are we having fun yet?