I have a TAdvStringGrid version 126.96.36.199 with Delphi XE3.
By default, the grid is configured to show gradient colors and gradient colors are set with grid.ControlLook.FixedGradientFrom,grid.ControlLook.FixedGradientTo.
If you want solid colors for fixed cells, set grid.Look = glSoft and then you can use grid.FixedColor to set the color.
This is also explained on the tips & FAQ page:
Nancy Lescouhier2013-06-20 08:08:49