Hi Wee Mei 

You could tell me that this is the normal appearance of PWBs from this supplier and I would have to believe you.  However, evidently you have compared it against other boards that you have received from this same vendor and you are seeing a difference.  I recommend asking the vendor to report to you why this lot of boards has a different appearance.  You should have been notified of any changes in the vendor's process.  That vendor runs enough of these boards that EITHER he can tell you why the change in shading on all of his boards OR he should recognize that you have purchased boards for a high reliability application and provide replacements for which there is no concern without needing further analysis first.

I am presuming that this is not a new vendor but one with which you do a regular business.

Glen