Why is the nose cone of SU-34 white?
It is all part of the camouflage. 

The blue parts on the bottom make the aircraft hard to identify from below against a blue-background (sky). The dark parts on the top make it difficult to identify from above against a dark background (ground). 

The white parts make the aircraft's size/shape difficult to determine. This helps to confuse the enemy by making the aircraft seem farther away than it really is.