Concept : Darken mode selects the Dark color as a result. Lighter Pixels are replaced and darker pixels do not change. This means when applied with the darken mode, Adobe Photoshop calculate the dark and light pixels of the both upper and lower layer. The dark pixels remained unchanged and the lighter pixels are dominated by the darker pixels. Hence the result will be overall dark because the darker pixels are displayed finally.

1. Open Photoshop. Create a new file and make a new layer. Keep it selected.

2. Open swatches from Window > Swatches

3. Choose Pure Red as foreground color. (1st col 5th row)

4. Select Rectangular Marquee tool and make rectangle on the new layer.

5. Fill the Pure Red color using Alt + Del. Deselect with Ctrl + D

6. Make another layer. Keep it selected.

7. Select Rectangular Marquee Tool and draw rectangle on the new layer such that it will overlap some portion of the bottom rectangle.

8. Choose Dark Red (just below Pure Red) as foreground color and fill the rectangle. Deselect with Crtl + D.

9. Make another rectangle on the same layer besides the Dark red rectangle. This layer should also overlap some portion of the Pure Red rectangle.

10. Choose Light Red (just above Pure Red) this time and fill the rectangle. Deselect with Ctrl + D

11. Change the mode of the Top most layer to Darken.

12. You will notice that the Dark Red remained unchanged and the overlapped portion of the Light Red rectangle is dominated by the Pure Red.