Here are some images that feature neutral colored shapes. Comparing the shapes in terms of color temperature will reveal how warm or cool they are.
The deck, for example, is much warmer than the shingled wall, but cooler than the yellow bumper.
This hay shed may not seem to involve neutrals at first, but if you tried copying it
you would see that with the exception of the tarp, everything in the picture involves all three primaries.
Subtle, i'd say. Watch out for texture. It wants to take over.