Why is the xfce mixer plugin icon sometimes blue or red?

[stozi]

Hi, I'm using aufkleber icon theme. I find that usually the sound waves coming out of my speaker in the panel icon for xfce4-mixer are red, but a couple times now when the computer has been booted up they are blue. I have done nothing. The blue icon is named audio-volume-03.png and red audio-volume-06.png. I could not find any clues in the default tango theme (since I couldn't find icons with exactly the same names) or on official xfce project pages.

please help me demystify this!

[ivy wang]

!!^^!!