There is a color model that represents a color by hue, saturation and brightness, where hue specifies the hue of red, orange, yellow, green, blue and purple, and saturation specifies the brightness of the color (the lower the value, the closer it is to gray). The mixing mode of "color" and "hue" of the brush is to modify the color with a similar color model. Color mode modifies hue and saturation at the same time, so when using a bright brush, the gray area with low saturation on the original image will also be dyed bright. In hue mode, only the hue is modified, and the gray part of the original image changes back to gray. This is why the tone mode looks darker.

To help understand, let's draw a gradient, a big red on the left and a gray on the right, and then use a blue brush (#00c0ff) to draw one in hue and color mode respectively.

After modifying the gradient with hue mode, keep the original saturation change, with blue on the left and gray with low saturation on the right. The "color" mixing mode changes hue and saturation at the same time, and the original gray also turns blue.

Of course, you can also try a very dull color (#a7ac9c, a little green and gray):

In Tone mode, the gradient keeps the original saturation change. Although the color of the brush is not bright, it can be seen that the big red part has turned into bright green. In the "color" mode, the whole painting is the same gray-green.