Neutral colors are the most commonly seen colors in nature or real life, and are a mixture of pure colors or a mixture of achromatic and chromatic colors. This study categorizes the mixed colors that exist between the basic colors that people perceive and examines the simultaneous contrast effect when they are adjacent to other colors. The subjects were presented with a variety of low-saturation color stimuli(R, YR, Y, GY, G, B, BG, B, P) that can be seen in nature and asked to select a color name. To this, the color stimulus that answered 'not all' was considered a neutral color. The effect of simultaneous contrast was investigated by selecting colors that look different when placed adjacent to various colors as neutral and primary colors. As a result, the neutral color category appeared in red and yellow colors, the primary color appeared most frequently in green color, and the achromatic color category appeared in blue and purple colors. It was confirmed that neutral colors were affected by hue and saturation regardless of brightness. Neutral color and primary color both show complementary color contrast, and when adjacent to yellow color, the strongest perceptual vibration occurs.Through this study, it is intended to clearly illuminate the category of neutral colors that are highly useful and to give efficiency to color planning through the characteristics to be considered when using neutral colors and other colors together.