how to make a control gets the backColor of a control below without be its child?

[st mnmn]

I have a userControl-'Board' which contains tow controls, let's call them: 'a' and 'b'.
'a' is located above 'b', but it's NOT its child!!!
'a' is 'Board''s child!.
I want 'a' to be transparent. so I wrote:

    SetStyle(ControlStyles.SupportsTransparentBackColor, true);
     this.BackColor = Color.Transparent;

and it is transparent, it got the backColor of 'Board'- its parent, but that not good for me!
I want it to get the backColor of 'b', which is below. so, I made 'a' as 'b''s child and that worked fine!, but slowed all the program.
my question is: does anyone know how to make 'a' gets the backColor of 'b' without be its child?