Concept
Grayscale mode uses 256 shades of gray with each pixel
retaining its brightness value from 0 (Black) to 255 (White). This
is different from desaturate command as it chages the mode itself.
Its no more RGB.
1. Open an image to apply photoshop Grayscale mode.

2. Image > Mode >
Grayscale
3. You will get the the grayscale image as follows.

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