Hello Hong,
The function 'whitebg' will change the background color of the axes, not the figure. See the documentation for 'whitebg': http://www.mathworks.com/help/releases/R2016a/matlab/ref/whitebg.html?searchHighlight=whitebg. Note that whitebg will be removed in the future.
To change the background color for the figure (i.e. the window) you can use the following command:
set(gcf,'color','white')
which will set the background color of the current figure to white. Here is a related MATLAB Answers post about changing the default background color: https://www.mathworks.com/matlabcentral/answers/96816-how-do-i-change-the-default-background-color-of-all-figure-objects-created-in-matlab