It's simple. Enter the channel, copy the green channel, select the green channel copy, and CTRL+L adjusts the color scale. Drag the black and white triangles on both sides of the input color scale to the middle (depending on the color of the picture), and the picture will become black and white.
Go back to the menu bar; Select load selection, and confirm that the menu bar selects reverse return layer.
Turn off the layer and activate the original layer copied by CTRL+J. good