, your idea about lights is great. Sometimes just flatout dark or an other color or a little bit of blur (or a mix of the three) work also. I am not a side-scroller veteran (especially not with 3d ones), mostly just played some Metal Slug and recently a bit of indie game Gigantic Army, here are some reference pics for color and brightness differences:
https://lh3.ggpht.com/uWrKdnWGdUE-tGWe_X976ALV6pVggiGS8OjMV2hDhGNPjOsQOODn-ejL3WF1Z0zrMdEy=h900https://thanksforyourplaying.files.wordpress.com/2013/02/ms2-x-hold-back-train.jpghttp://nyu-media.com/wp-content/uploads/2014/03/gigantic_army_onsteam.jpghttp://d2oah9q9xdinv5.cloudfront.net/cache/images/games/1/30/29039/crop_640x480/GA_314.jpgThough from the sound of your recent additions it probably already is alot better, futher additions are like cherry on top.
