What do shadows, midtones and reserved lightness mean in PS color balance?

In the color balance of 1 PS, shadows, midtones, etc. Indicate the main range of color cast adjustment. If the shadow is checked, the color scheme will be dominated by dark parts, and then the middle and bright parts will be changed. We usually judge by images. For high-profile images, the brightness is often slightly white, the color saturation is very low and the color performance is poor. We will focus on the parts with higher color saturation (which parts are brighter and more expressive, you need to judge for yourself), and generally adjust the middle tone or dark tone. On the contrary, if you encounter a low-key picture, focus on the middle or high-key area.

2. Keep light and generally check. Because color balance is a color matching based on the principle of reverse color. RGB exists as additive mode. When you drag one or more sliders to the right at the same time, they all tend to be white, which makes the image brighter. If this option is selected, only the difference of each monochrome adjustment will be adjusted, regardless of synchronous brightening.

For example: red +20, green +20 and blue +30. If "Keep Brightness" is not checked, it means that the whole image is 20 bright and blue is 10. Check "Keep Brightness", the overall brightness remains unchanged, but the blue color is turned off 10.