Just wondering why Construct2 is not shipped with a Chroma effect. I've made a quick-and-dirt chroma effect by altering the replacecolor effect to work on the alpha channel.
The effect is called "chroma2" since I did a previous, failed version which replaced colors with background object color.
<img src="http://dl.dropboxusercontent.com/s/5ealf0oqaz0qlz5/chroma_effect_screenshot.png" border="0">
You could use this plugin to make quick-and-dirt solutions for chroma "Photocall" parties. This would require some kind of motion picture support, also.