There is a similar bug report for KAlarm on Gentoo (KDE bug report 122988, http://bugs.kde.org/show_bug.cgi?id=122988). There, the icon background colour is whatever is currently set as the KDE window background colour. It would be interesting to know if the KNotes icon similarly changes colour if you change the window background colour setting.
If the two bugs are actually related, it would suggest that some underlying KDE configuration bug is the cause. -- David Jarvie. KAlarm author & maintainer. http://www.astrojar.org.uk/kalarm
