Do 18 shades of gray and neutral gray mean the same thing?

It means the same thing. The reflectivity of the neutral gray color is 18%.

Neutral gray in RGB color mode refers to R:G:B=1:1:1, that is, the three colors of red, green and blue have equal values, which is neutral gray. When R=G=B=128, it is called "absolute neutral gray".

Grayscale is usually expressed as a percentage, ranging from 0% to 100%. Only integers can be entered in Photoshop, while decimal percentages are allowed in Illustrator and GoLive.

This percentage is based on pure black. Just the opposite of RGB, the higher the percentage, the darker the color, and the lower the percentage, the whiter the color.

Extended information:

Grayscale in painting

The word "grayscale" was originally a term for black and white photography. It means that due to the different colors and brightness of each point in the scene, each point on the black and white photo (or black and white image) taken shows different depths of gray. Dividing black and white into several levels is called "grayscale".

The more gray levels that can be presented, the richer the levels of the picture will be. It is very appropriate to use the word "grayscale" to apply to realistic pen drawings that are created using only black carbon ink and reflect their unique grayscale.

A term with similar meaning in visual arts refers to "brightness". In order to illustrate the importance of grayscale in realistic pen and ink drawings, the meaning of lightness should be further explained and understood from the perspective of painting.

Lightness refers to the degree of darkness. The daylight under an incandescent lamp is the brightest in brightness, and the black in the lamp shadow is the darkest. Between these extremes of lightness are various intensities of gray. It is difficult to find pure black, pure white or pure gray in the natural state, because any object surface has its own color, and this color will be affected by the color of light (or natural light). However, no matter what color, objects always have relative lightness and darkness, and this is the lightness that can be captured and recorded by painters.

Baidu Encyclopedia - Neutral Gray

Baidu Encyclopedia - Grayscale