The change happened in 5.00 when the default color map is applied to a surface. The default map is internally defined and not editable by the user or a file. We have been revising the graphics engine and I have not been able to determine if this was intentional or not. If the surface is an isopach it gets the correct color mapping.
I have added an enhancement request to make the default color map user definable. As the user can now create their own or import others, this will have a low priority.
tim