There is recent research being conducted right now at UCDavis that has shown that birds prefer colors like red and orange over blue and green. Orange and red foods in the wild contain carotenoids which are a source of antioxidants and vitamins. They also give pigments to the feathers of birds that are red, orange, and yellow. It could just be that Bubbles is simply eating what evolution has dictated him to. I don't think this is an unhealthy eating habit, although I do think you should try to get him to eat other things like broccoli, kale, etc. There are many ways to do this including eating them in front of him, mixing them in his food like Jessica said, rolling a wet piece in seeds/pellets which can entice him to try them. Darwin will not eat kale and broccoli if I just put it in his dish, I have to hold it for him to eat it too, so maybe that's just what you have to do! He always goes for the yummier stuff first
If you think he isn't eating enough greens, switch what you give him. One day give him the orange stuff he likes. The next day hold him and get him to eat a good amount of greens and don't give him the orange stuff that day. That way you can try to control what foods he is eating.