I took an image of the Crab nebula in a set of all red filters over the past two nights.
By all red filters, I mean the filters I used pass emission lines that are red. The three lines are [SII], [NII] and Ha. I have filters for each of those emission lines.
Normally when someone shoots the Crab as an RGB shot, the Halpha line and the singly ionized Sulfur and Nitrogen lines are all blended together in the red channel. That is because their wavelengths all fall within the red portion of the spectrum (673.4=[SII], 658.3=[NII], 656.3=Ha).
To make this tricolor image, I combined images taken through my [SII], [NII] and Ha filters into a single tricolor image.
So instead of seeing these three emission lines as slightly different shades of red, we see a tricolor image revealing contrast not normally seen in an RGB image.
http://www.rdcrisp.darkhorizons.org/m1_crab_nebula_S2_N2_ha_page.htm