A Theory Concerning Leland's White Hair

"We were told that BOB the killer was a gray haired man. When Leland killed Jacques, his hair turned gray overnight. Leland said when when he was a child, the gray haired man who lived next door was named Robertson." - Agent Cooper, episode 16 (2.009, Arbitrary Love).

Cooper's explanation for Leland's changed hair color has always seemed like an afterthought rather than a well-planned decision by writers. Attempting to connect Leland's whitened hair to BOB prompts more questions than answers. As someone once asked me, why didn't his hair change after Teresa Banks' murder? I've always assumed his hair changing hue overnight was associated with his killing Jacques, as Cooper states. If the whiteness of his hair was caused by BOB/Leland's murder of Laura, one would think his hair would have changed color the morning of episode one - yet it remains brown throughout season one. Furthermore, Leland seems to be reborn with Renault's death. In the script for episode eight, Leland has the following exchange with Doc Hayward: 

DOC

As your doctor, Leland, what the hell happened to your hair? 

LELAND

Isn't it strange? I woke up this morning, looked in the mirror and there it was, literally changed overnight.

DOC

Considering all you've been through ...

LELAND

It's a funny thing, though. Seeing it I seemed to realize I'd turned a corner somehow. I felt a great deal of sadness still, yes, but it wasn't overwhelming; I actually physically felt as if a large weight had been lifted from my heart.

Leland of season one, often depressed and prone to emotional outbursts, suddenly is filled with joy. He bursts into song instead of tears. He no longer begs for a partner with which to dance; he leads. 

In terms of trauma being the cause of his changed hair; there is also the trauma of his childhood abuse. The possibility of Leland enduring the sexual and mental abuse later inflicted on his daughter. To live with the presence of BOB is to endure much. No matter how distant a shadow he remains. One would think his hair would have changed considerably earlier. 

The murder of Jacques Renault was different in that it was of Leland's own volition. I believe the shock of killing by his own hand turned his hair white overnight. If Leland's hair turned white by the death of Jacques Renault, why, as someone asked, didn't it change with the murder of Teresa Banks? The only answer that arrives in my mind is in accordance with Leland's dying words, "Oh God, they had me kill that girl, Teresa."