Why are these apples green?

In order to answer this question, I'll mix in a bit of plant biology with basic physical science light interactions.  
Plants contain various pigments.  Students generally learn about one specific plant pigment.  That pigment is chlorophyll.  Like any pigment, chlorophyll absorbs light.  White light is made up of red, orange, yellow, green, blue, indigo, and violet wavelengths of light.  The chlorophyll pigment will absorb all of those colors except green.  Green will be reflected.  That reflected light travels to your eye, stimulates the corresponding cones in the retina, and you see the color green.  The apples that are pictured are green because they are reflecting green light due to the pigments in their skin.  
Chlorophyll is not the only plant pigment though.  More than likely, those apples are filled with anthocyanins which are plant colorants that are responsible for the red, purple, and blue colors displayed in many fruits, vegetables, and flowers. 
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1082903/