This image uses the same luminance, red, green and blue filter data as the other images of M57, but in this case the data from narrow band filters had been added. Narrowband photons are emitted at specific wavelenghts by atoms undergoing quantum energy state changes. Specifically this images uses hydrogen alpha, oxygen (III), and sulfur II narrowband photon filters; hydrogen alpha due to the change in energy state of hydrogen atoms, OIII caused by atomic collisions with oxygen ions, particularly at high temperature, and SII, again collisions with sulfur ions, particularly at higher pressures (apparently).
In any event, while the chemistry is fascinating, narrowband data carries along with it much detail on the structure and composition of the image subject, that allows for more refined images. However, a drawback is that they are difficult to represent within the broadband colours we are most used to seeing. This becomes an image processing challenge.
I hope you can see from this image that much more detail is apparent in the final image through the incorporation of narrowband data. (also in the gallery, M42 incorporates both broadband and narrowband data, while NGC2024 incorporates only narrowband data.)