SIMPLE DUOTONE EFFECT ON PICTURE
Prepare your image in Adobe Photoshop and convert to Greyscale.
Apply a gradient map to your image, with a solid dark background behind your image. Edit the gradient to suit the color you intend for your duotone effect.
Apply a gradient map to your image, with a black-white gradient background behind your image. Edit the gradient (of your gradient map) to suit the color you intend for your duotone effect.