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How many colors do you see when a colored disc is rotated at high speed?
I asked ten people to answer that question.

Most people refer to the seven rainbow colors that make up a rainbow as “red, orange, yellow, green, blue, indigo, and purple.” I wondered what color the rainbow would look like if I painted the seven colors evenly on a disc and rotated it.

I read in a book that mixing paint colors makes black, and mixing red, green and blue LED lights make white. I asked 10 people around me. The answers were varied. I decided to try an experiment and here is what I found out.

10 replies

The majority of respondents answered black for four people, followed by white for two people. Each respondent's answer is summarized in Figure 1.

Figure 1. Summarized replies from 10 people

Hexadecimal color code

First, a disk with seven colors evenly painted on it is created on the computer and the color printer. Some people are simply red, some are light red, and some are dark red. To eliminate individual differences, the seven colors are represented using hexadecimal color codes. The color codes can be found by searching the Internet, for example, "red hexadecimal." Figure 2 shows the hexadecimal number of each of the seven colors found on a computer in this manner.

Figure 2. Rainbow colors against hexadecimal color codes

Enter the hexadecimal color code searched on the Internet as a parameter in the Hex(H) input window shown in Figure 3, and click the [OK] button in the upper right corner to draw that color.

Figure 3. Making seven rainbow colors by entering hexadecimal color codes

Making a rainbow-colored disk with a cardboard

Divide the disk into seven equal parts as shown in Figure 4 and paint each area with the seven rainbow colors. Print this in color on a printer, glue the printed material to a piece of cardboard, and cut it into neat disks with scissors. Depending on the printer, the color saturation may vary slightly, but I will ignore that and proceed.

Figure 4. Making a rainbow-colored disk with a cardboard

Making a rotating device

Use a DC motor for a plastic model. The motor I used is a DC 3 V motor. The motor, battery box, and electronic circuit to control the rotation are mounted on the board. If you just want to rotate the disk, you can do it quickly by connecting the batteries directly to the motor. However, as this is a monthly FB News article, I will include a little bit of electronic circuitry. By turning the knob, the rotation speed can be changed continuously. The device is shown in Figure 5.

Figure 5. Device to rotate the disk

Now what happens when you rotate it?

As I turned ON the device and rotated the control knob, the rotation speed gradually increased, and I could see the colors of the disks changing. At low speeds, I could follow the seven colors with my eyes, but as the rotation speed increased, the colors appeared to mix together. The colors were slightly different from those seen indoors and outdoors.

Figure 6. Indoor shooting (shutter speed 1/10)

Indoors the color appears light brown with a light reddish tint. (Figure 6) Outdoors, the color is slightly lighter overall than the color seen indoors, but not as "white" as some people's answers.


Like most color designers, I have never studied color in detail before, so for me, the results of my experiments are all I have to go on. However, there may be variations in the way the experimental apparatus is used, the quality of the disks, or the color variations of the printer. There should be a theory that says, "This is how the experiment turned out, but ideally it should look like this."

I know that the monitor screens we use are microscopically composed of RGB (Red, Green, and Blue) luminous elements, and that when each is fully lit, they become “white.” I conducted an experiment based on this assumption, but it did not turn "white." This may be because the seven colors painted on the disk are not light-emitting elements.

Experiment again with the three primary colors red #FF0000, green #00FF00, and blue #0000FF

A disk filled evenly with red, green, and blue became blackish when rotated at high speed. It was observed that the exact color appeared dark gray indoors and gray outdoors, although slightly lighter than indoors. (Figure 7)

Figure 7. Experiment again with newly made disk

Theoretically, it seems to be black, but the author's own speculation is that it appears gray due to a mixing of two phenomena: one is that light from nature is reflected on the surface of a disc printed by a printer, and although the luminous material becomes white when mixed with the luminous material, it becomes black when mixed with paint.

In any case, when the disk produced by the above method was rotated, it became neither white nor black, but rather dark gray in the actual experiment.


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