the color of instance label in symbol editor

Recently I found some instance labels were in color white.  But if I create a new one, the color is yellow. And I haven't change system LPP. In default, the instance label uses color yellow. If I change the type of white label  to other than "instance label" and then change it back to "instance label" again, the color changes to yellow and never back to white. How could this be possible?

Thanks.

Fred