Piston darker than usual when trying to mine/break it.

When trying to mine or break a piston it becomes a little darker which doesn't seem to be the case for other blocks. 

How to reproduce

Start by crafting a piston
Than try and mine the piston using you're fist or using a pickaxe, it will change color.

Observations: Piston changes color once you try mining it.
Expectations: The block shouldn't change its color even when being mined.