Color seems like one of the most immediate things in the world. Grass is green, the sky is blue, blood is red, snow is white. It is natural to think that color is simply a property sitting on the surface of things, as if objects carried it in the same direct way that they carry mass or shape.
The situation is subtler than that.
Color appears when light interacts with matter and then reaches an observer. Neither the object alone nor the light alone is enough. What we call color is the result of a relation between the two, and also of the conditions under which the object is seen.
Light comes in different wavelengths. The visible part of the electromagnetic spectrum spans only a narrow range, but even within that narrow band different wavelengths are registered by us differently. When we speak of white light, we mean light that contains a broad mixture of visible wavelengths rather than strongly favoring just one part of the spectrum. Sunlight is close to this, which is why it often serves as the natural reference point.
When white light falls on an object, some wavelengths may be absorbed more strongly than others, some may be reflected, and some may pass through. The light that finally reaches our eyes is therefore not the same as the light that arrived.
This selective interaction is what gives rise to color.
A red object under white light appears red because it reflects red wavelengths more effectively than others. A leaf appears green because the molecules in it absorb much of the red and blue light while reflecting more of the green. A black object absorbs much of the visible light that falls on it. A white object reflects a broad range of wavelengths rather than strongly preferring one part of the spectrum.
This already shows that color is not simply painted onto the world in a self-standing way. Change the light, and the color changes. A white shirt at noon, under a warm lamp, or beneath a colored sunset is not receiving the same illumination, and it does not send the same light into the eye. The object is the same, but the perceived color can shift.
There is another subtlety as well. The eye does not read out wavelength one by one like a spectrometer. Human vision is based on a small number of different kinds of light-sensitive cells, each responding more strongly to some parts of the visible range than to others. The brain then compares these signals and constructs the experience of color from them. This means that color belongs not only to the interaction between light and matter, but also to the way our visual system registers that interaction.
In that sense, color is real, but it is not fundamental in the same way as wavelength is fundamental. Wavelength is part of the physical description of light itself. Color is what certain ranges and mixtures of wavelengths become when they are processed by a visual system like ours.
This is why the same physical light can look different under different conditions, and why different mixtures of wavelengths can sometimes produce the same perceived color. It is also why the world is not literally colored in the way a child’s drawing is colored. The world contains matter with certain structures, light with certain wavelengths, an environment that affects how light is distributed, and observers with certain sensory systems. Color arises from the meeting of all four.
At the microscopic level, the reason matter absorbs some wavelengths and not others lies in the quantum structure of atoms and molecules. Electrons in matter can only occupy certain allowed states, and transitions between those states require specific amounts of energy. Light whose energy matches such transitions can be absorbed efficiently, while other wavelengths may be reflected or transmitted instead. The colors of the world are therefore tied, in the end, to the quantum structure of matter.
See also: What quantum physics changed