Blue is the color of the sky and the ocean. Green is the color of grass and leaves. They are colors that act as the backdrop for all other colors found in nature. Everything looks good next to them- and isn't that essentially the definition of a neutral? Need proof? Here you go- we'll go right through the colors of the rainbow (ROY-G-BIV):
These wild roses look pretty great next to both the afternoon sky and the green leaves around them.
These once-green leaves have mostly turned orange and look at how lovely they are next to the still green leaves in the middle? Gorgeous!
Peek through these orange leaves to the bright blue sky and you'll find it's just as lovely a combination as orange and green.
Just like the roses, these sunflowers look lovely next to both their green leaves and the bright blue summer sky. At this time of year, there's not much that's more cheerful than a sunflower.
GREEN and BLUE:
The thing about neutrals going with everything is that it includes each other. Just like you're allowed to put black and brown together, the same goes for blue and green.