You've read and have employed the following:
You can also use shades of gray.
A lighter shade of gray blocks less.
A darker shade of gray, blocks more.
|This Shade . . .||. . . Blocks this Much of the Effect|
You can choose shades of gray in the Color Swatches panel.
Click Color Swatches in the panel bin on the right side of your screen.
If it's not there, go to Window > Color Swatches.
Masks are monochrome.
Don't use colors with masks.
The Background copy layer of the grass photograph below was converted to black-and-white (Enhance > Convert to Black and White).
A mask was created in the Background copy layer by clicking the Add a mask icon.
The mask was brushed with the Brush tool with black and three shades of gray.
Black blocked 100% of the black-and-white conversion.
Brushing with the three shades of gray blocked less and less of the black-and-white conversion.
Below, look for the mask in the Background copy layer.
Here's the mask enlarged, with the photograph.