I immediately thought the green background must be the color of the leaves,

yes, naturally strawberry's leaves are green, that certain one was a dried diseased thats why it isnt green.

Sigma 150mm is a good lens, with a nice bokeh. not the best one out there, but I used to it and ow I like it, sometimes it becomes my everyday choice, cause it is good for capturing #streetphoto and city life of the streets, which is my other fave subject to shoot.