Ever noticed that TabPages added to TabControls in WinForms become transparent, effectively making all child controls display with a white background (same as the TabControl itself)?
Ever had it where controls randomly decide that they're not going to play ball and display using the default control colour (that lovely grey)? The TabPage checks the background colour of subcontrols are one of the default colours called Control, and even if that's what's set for the BackColor property on your control it may not be enough. It seems that it has to be the "Default" Control colour which you can reset to by selecting the value of the property and then deleting it.
Voila! The control reverts its' BackColor to the super special "Default" Control colour and everything's groovy again.