Chemistry? Optics? wtf?

One day, Degen noticed a rather peculiar phenomena.
As he stood by the coffeemachine at work, he watched the lifegiving black liquid pour into his cup and the white milk being added. The drink now held the beautiful brown/tan color that he was used to.
Degen returned to his workstation, sat down and took a sip of the coffee, and then looked down into the cup.

The coffee had changed color to something greyish, and did not look nice at all.
This intrigued Degen, and he began to study the effect day after day.

Well... wtf is going on? Is there some molecular change that happends on the way over, perhaps by passing the smelly guy in the CA department?
Or is it perhaps the lighting that is different in the two settings?
Could it be that someone spikes Degens coffee everyday?

Unfortunately, we might never know...