Why does lead nitrate powder on heating starts turning yellow from the top rather than from the bottom?
Lead oxide is a peculiar type of salt. It's brown when it's hot and turns yellow when it's cool. The lower most part remaining white could be due to two possible reasons:


The top-powder becoming yellow indicates the region is more cooler than the salt with is present at the lower part of the test tube. The lower most part is in direct contact with the flame and the  upper part is receiving heat due to conduction and losing heat to the atmosphere; many factors could be at play here.
Another possible deduction could be the paucity of atmospheric oxygen at the bottom of the test tube than at the top and hence the rate of conversion to lead oxide to the top would be faster than at the bottom.




In case you haven't done the experiment and want to see what the OP is trying to convey, here's a youtube video that demonstrates the whole lab-based thermal decomposition of lead nitrate (it starts at 1.32  incase the timed setting of the link fails):
http://www.youtube.com/watch?v=csEnz9ZsAFM&amp;t=1m32s