How to whiten buttercream? Any idea and tips? 

Try using clear vanilla and then add white gel coloring.  Americolor white works well.  I add the white coloring while mixing so I can be sure to add enough.

When I make my AMBC I use shortening and the whitest butter I can find.  Like cutiger said, I use clear vanilla.  My buttercream always has a slight yellowish tint to it.  This is because of the butter.  I use a toothpick and add a TINY amount of violet food coloring to the buttercream.  It brings it back pretty close to white.  If I need it more white I then add white icing color to it.  If you want it WHITE....don't use butter.

The tiniest of tiny drop of blue color should help whiten up off color b'cream.  However, I see no need unless you have used something different.  I have made b'cream for over 30 yrs and never had to whiten it.===== oh, there was one bride who had the bluest white cloths on her  sun drench, outdoor tables and was a bit unhappy w/my b'cream but that was one out of 1,000s.