Gray is the color of neutrality, "neither subject nor object, neither inner nor outer, neither tension or relaxation." Gray feels as though it is not colored, not dark, not light - a separation between two distinct entities, a demilitarized zone free from stimulus. Gray communicates an element of non-involvement or concealment. It's a color that remains uncommitted and uninvolved.
This is the color of calmness, repose and unity, symbolically the color of sky and ocean. Looking at blue relaxes the central nervous system - blood pressure, pulse and respiration rate all go down, which allows regenerative systems in the body to work on healing. When folks are ill, the physiologic need for blue actually increases! The physiologic associations with blue are those of tranquility. The psychological associations are of contentment, gratification and being at peace.
Where red stimulates, yellow suggests. It can elevate body rates as red does, but its effect is less stable. Yellow is primarily the color of happiness, cheerfulness, expansiveness, lack of inhibition. It is the welcome warmth of the sun and the glow of a spiritual halo. While calming and relaxing, the color does suggest a desire for change, that things are never quite at rest - people who favor yellow may be very productive, but that productivity often occurs in fits and starts.
Orange -- poor oragne was neglected. Nothing here on orange. [much later]: While orange may suggest fire, vitality, warmth and energy, all lovely associations, it’s the color most-detested by Americans (it is more popular in Europe and has particularly strong appeal in Latino and French cultures). Maybe it’s the 70s associations you guys are stuck on. But the research suggests if you’re going to use orange, it’s best tolerated when you are evoking a natural association. Like carrots. That, or go for a deeper orange that is earthier.
[Much Later] -- Grok follows up with some more practical uses of colors.