I'm not a Construct 2 user, just happened to look around here, but maybe I can help.
Do you mean dithering (http://upload.wikimedia.org/wikipedia/commons/6/6d/Dithering_example_dithered_web_palette.png)?
I bet the easiest way to create this would be to just create a dithered sprite, for example by exporting a GIF with dithering in a graphics editor like Photoshop.