Our PushButton uses ControlPaint.DrawButton to draw the button, and this method does not support background coloring (other than what you might get from a Themed OS.). So, for now, there is no property setting that you can use to handle this.
Some ways around this would be to add a Windows Forms PushButton (which does accept a backcolor) to a cell using the Control celltype. This would only be useful if you had 1 or 2 cells where you wanted to see this button.
Here is a little sample.
If you needed to have this button in a lot of cells, you would have to derive a custom button to handle this. You could either do a bitmap button like the one shown in grid\samples\in depth\cellbuttons or you could derive a cell control like the grid\samples\cellstypes\calendarcell but using a Windows Forms PushButton instead of the calendar.