Mustard color is normal. Dysentery/Nosema is dark brown to black.
Great topic, I've been thinking about this all week. From what I've read, there is no definitive field diagnosis for Nosema:
http://department.caes.uga.edu/entomology/bees/Disorders/Nosema.htmI have fecal matter on my protective gear that is certainly not yellow in color, but the color of Ochre. Somewhere between Ochre and Brown on this
http://en.wikipedia.org/wiki/Ochre page describing various shades of brown.
Apparently, one of the simplest ways (short of having a microscope and macerating your bees in some sort of solution and looking for the actual culprit) to definitively diagnose Nosema is stated in the first link in the second paragraph.
I hope that helps!