Different PCB color (is it a different material?)
The green areas have been coated in solder-mask.
The yellowish area is simply the base color of the PCB material, which has not been coated.
It's the same base material throughout.  

You would typically achieve this in your designs by adding features to the solder-mask layer(s) of your board.
The solder-mask layers are usually 'negative' images, so drawing something on those layers will result in the solder-mask being left out in those areas on your board.