Step 1. Creat new file and apply Linear Gradient as it's shown on the image below.

Step2. Then apply Filter/Render/Diiference Clouds and press CTRL+I

Step3.  Then press CTRL+L. Whe the window opens place the middle traveler to the right and adjust the contrast by the left traveler. Press Crtl+U, tick Colorize and set the following data: Hue 225 ; Saturation 25 ; Lightness 0

That's it then!