the datagrid columns backcolor did not take effect on the for load if the form is a subform for a form that have an mdiParent

ex.

drugsDD=new drugs();
DD.mdiparent=this.mdiparent;
DD.show();

the datagrid columns in thedrugs form have its default backcolor and made no change.
but if i comment the line"DD.mdiparent=this.mdiparent;"
the columnbackcolor changed according to the code in the form load.

what is the reason?????????????