How do I reset a .NET Windows Forms TextBox BackColor property?

If you mean in the designer, you can just click into the BackColor property and delete whatever is in there and then press enter. This will reset the property back to its default value.

If you mean in code, you can set the BackColor property to Color.Empty, and this will have the same effect.

TextBox.ResetBackColor()

It doesn't pop up in IntelliSense for some reason, but it's there and it compiles.