Go to Edit > Preferences in the main GIMP window. The Preferences window will appear.
Expand the Image Windows options on the left side of the window by clicking the little arrow thatís next to it. Choose Appearance.
Choose Custom Colour from the drop down menu next to the words Canvas Padding Mode on the right portion of the Preferences menu
Make sure the Custom Padding Colour is white. If the little box is not white click it and choose white from the color options. Click OK.