Here's my method . I use GIMP ( GNU license , this means you can use it for free ) . In the color selector push the buttons like in the image ( push de slider down until it's black like in the image ).
Take a screenshot , then "Ctrl+v" in Paint , select the gradient and save it as BMP ( if you save it in JPG now you'll lose quality ) .
Open it in GIMP again , click on "Colors" => "Colorize" and then push the sliders until the desired Color(Hue) , Saturation and Lightness.
Save it and you can use like that , or you can customize it
.