Images from SOHO, SDO and other sun observatories are often coloured differently for different wavelengths or temperatures:
http://www.nasa.gov/mission_pages/sunearth/science/Sun-Wavelength-Chart.html has statements like "This light is emitted from the upper transition region and the chromosphere and are typically colorized in red."
How is the convention of which temperature or wavelength gets which colour established?
Is it simply so that scientists can immediately tell what temperature or wavelength a picture shows, or are the colours optimized to show particular features?
And why is there a white ring or circle in the sunshade in some SOHO imagery, and why different sizes compared to the disc in the two photos above?