Note the difference in color between this [old commodore board](http://myoldcomputer.nl/wp-content/uploads/2015/11/KU-14194HB.jpg) and this [newer board](https://www.autodesk.com/products/eagle/blog/wp-content/uploads/2017/03/shutterstock_446089405.jpg). It might seem like lighting, but after looking at my own collection of scrap boards, the older ones tend to have a much darker color than the new. I thought it might be aging on the mask, but all the old ones essentially have the same color, so I thought it unlikely. 

Does anyone know the reason for this color change? 

Is it a change in board material, masking chemicals, etc?