Colour Change of Phyllite

The interior is dark grey while the exterior surface is light grey and whitish, why? 
It cannot be every time you drill, it hits the quartz veins.

It is due to the exposure to water, sunlight and atmosphere which result colour change. 'Therefore, immediate examination and record should be done as soon as the sample is brought up from deep below, not after one or two months when the samples are brought back to the office.
However, to Engineers, colour change is not an important characteristics. They are more concerned with the rock sustainable structure and strength.