# Can paint colors be classified by wavelengths reflected or absorbed?

As I understand it, we recognize pigment colors when certain wavelengths are reflected while others are absorbed. Since astronomers study star composition by their spectrums, could every paint color be given its own classification with a similar method?

I realize it would be terribly inconvenient, and maybe interfere with people's creativity... but is it possible?

• Ever had paint mixed? The mixing formula is essentially a color code. – MaxW Jul 24 '18 at 20:14

Yes, kind of.

The problem is that paint colors are made of pigment. As you say, some wavelenghts are reflected and some aren't.

However, the color perceived depends on the ones that come back reflected towards our eyes. Now: what if you don't send all wavelenghts? The perceived color will be different. This means that the actual color depends on the light you throw to it.

So actually colors of objects are represented by the reflectivity, $\rho(\lambda)$, that is the percentage of each incident wavelenght that the object does reflect.

Like this, you can classify every color regardless of the ambient light you are using (spectrum and intennsity).

For example, if you use XYZ coordinate system, which is easily convertible into RGB, is

$$X_i = \int_{visible} \rho_{obj}(\lambda)\cdot I_{light}(\lambda) \cdot \bar{x}_i\cdot d\lambda$$

and $\bar{x}_i$ can be found in tables.

In short, reflectivity is like the spectrum of a paint. It determines is "color" independently of the incident light.

To compute the perceived color coordinates, you must specify the light you are using.

PS: When I read Paint in the title of the question I though about the computer program "Paint". I should spend less hours working here haha. But that makes me say that light colours on a screen would be very different: actual light sources do have a true spectrum. However, pixels have it reduced to a sum of three tiny spectra.

It is more than possible- it is in your local paint store right now. Most paint stores that do custom color blending have a spectrophotometer connected to a computer-controlled set of mixing pumps. the spectrophotometer measures the reflected wavelengths from a sample of the desired color and a program running on the computer translates that into mixing pump commands so as to blend up a fresh gallon of paint that is a nearly-perfect color match to that on the sample.

In principle, yes: each color perceived by the human eye can (in most cases) be represented by three numbers corresponding to the red, green, and blue response to the color. Most people have red, green, and blue photopigments in their eyes. Some people have only two of the photopigments; some people have hybrid photopigments. If we discount those exceptional cases of people who see colors differently from most people, each paint color can be represented by a three-number code.

However, paints have other properties that influence their perceived color: transparency, "orange skin", and glossiness are three such properties.