Well Seinar Fleet does develop TIEs in mass in a variety of facilities so it's possible that construction material would not be uniform through out different manufacturing facilities. As a matter of uniformity and symbolism however, having them that same cold military gray makes them more menacing...

It's kinda like the immortals in 300. You kill one but another quickly takes it's place, a never ending horde of destruction that the enemy would feel exhausted at fighting the same enemy over and over... Psychologically it makes more sense for the Empire to drain the morale of a enemy force to get in their head that they can not win. That's why ISDs and SSDs like the Executor were built at such an massively large scale... Intimidation to the point of surrender without a fight saves the Empire resources over all. The Empire plays under Tarkin's doctrine of "Rule by fear and terror"... Making them an uniform color makes sense. =D (I know, lots of EU there... but it makes sense eh? ;) ).