Random colors disapear from the palette.

Hello there,
I have come to a little problem, when I draw at some point a color from the palette disapears making the vector shapes of that specific color become the default color (red). Any idea if this is happening from something I do? Once a color disapeared when I moved a new created color up in the list, another time a color vanished in an Undo action.
Any suggestions are welcomed.

Thank you very much,
Vassilis