Yep - do everything you can to resist the urge to paint that awesome deadly scene at night. Don't line up those vampires and werewolves on opposite ends of a Roman stadium and fill the seats with ghoulish spectators dressed in pink and green sequins (or whatever). Of course, your hero enters the scene at the fifty yard line and sees all the fine details. It is an awesome scene, designed to make the brave quake and the romantic quail. Of course, his lady love is going to be ripped limb from limb if he doesn't save the day, or rather the night.
Okay so that is a totally over-the-top scene, but you get the idea. Paint your scene to the hilt, and then step back and consider just exactly what your hero can see. Consider the lighting. In this case, night time. Most colors at night are washed out to shades of gray, though there might be something more around a campfire or by flashlight. If you aren't sure, do a little experimenting. Go outside during the middle of the night, and if the city night isn't dark enough, take a drive until you run out of streetlights. Pick a full moon night, just so you know. Take my word for it, a new moon night is very dark; you'd be doing good to see your hand in front of your face.
Once you have the lighting and coloring down, the next thing you need to consider is how much of your fabulous scene your character actually can see. Is there any artificial lighting? Add some torches around your stadium to show it up a little, but be careful of placement. Torches set around the field, if not carefully shielded, will dazzle the eyes of your spectators and they won't be able to see the bloodshed. So you shield your lighting so it spotlights the field; that means your hero can't see the people in the stadium. He might be able to see black silhouettes but I seriously doubt he'd be able to count them.
Oh, but you really love your scene in all it's macabre details. That's fine, all you need to do is find a way to walk your character through it all. Parade him around the stadium in front of the audience. Evil people like to show off all their power and glory; it can really cut into the self esteem of your hero, making his challenge that much more daunting. It also gives us all an up-close look at all the fine details of your sizzling scene, and don't forget, all that showing off can really puff up your bad guys. There might be a tiny shred of advantage in there somewhere.
Now, your hero is standing in the middle of the stadium with vampires gathered on one end and werewolves gathered on the other end. We have lit the grounds pretty well, but now the stress has greatly increased. We all know they are mortal enemies but your hero is the focus of attention and his lady love is stretched spreadeagled in the very center of the field. What is your hero's next move? I don't think he's going to count his opponents, nor is he going to pay much attention to what they are wearing, or not wearing, as the case may be. Yeah, he's going to see them. He's probably going to glance at the werewolves and say something like, "Christ, there must be twenty of the bastards." And then he'll glance at the other end, and maybe do a double take, and say something like, "Are those suckers wearing purple?" Then he's going to concentrate on trying to get his girl untied before they can close on him and shred them both, but that's your part of the story.
Have you ever painted a scene only to discover there's no way anyone can actually see it? What did you do about it? Let me know; maybe you have a trick or clue, I haven't thought of. Share a little; it feels kinda nice.