I use these in the plural because the shadows of both are innumerable, and because all, except perhaps turquoise and apple-green, are natural shadow-tints.

Green and blue can be used together or separately, according to the skill and what is called the "colour-sense" with which they are applied.
To use them together requires not only observation of colour-occurrences in nature but sensitiveness to the more subtle out-of-door effects, resulting from intermingling of shadows and reflection of lights.

Well done, it is one of the most beautiful and satisfactory of achievements, but it may easily be bad by reason of sharp contrasts, or unmodified juxtaposition.
But a room where blue in all its shades from dark to light alone predominates, or a room where only green is used, bright and gray tones in contrast and variation is within the reach of most colour-loving mortals, and as both of these tints are companionable with oak and gold, and to be found in nearly all decoration materials, it is easy to arrange a refined and beautiful effect in either colour.
It will require little reflection to show that a hall skilfully treated with green or blue tints would modify the colour of sunlight, without giving a sense of discord.

It would be like passing only from sunlight to grateful shadow, and this because in all art the actual representation shadow-colour would be blue or green.

The shadow of a tree falling upon snow on a sunny winter day is blue.