Concept: Channel mixer is suppose to be one of the best photoshop option to get grayscale image. This time we will work with all RGB channels.

1. Open an image.

2. Image > Adjustments > Channel Mixer

Select Monochrome. The output Channel will turn to gray.

By default you will get Red 100%

For the next image, try Green 100% and make red and blue 0%, similarly go for Blue 100% and change the values of Red and Green 0%

Each time go back using History palette to the original color image and apply new fresh command.

3. You will get the following result 100% Red.

4. Result 100% Green.

5. Result 100% Blue.