Washed out images

The plugin is working as expected with PNG images, but jpegs often produced washed out colours, especially when using colours that aren’t high contrast. For example, I’m trying to make an image using a dark blue (#31355d) and a light red (#ef4125).

Here’s the original image:

http://i.imgur.com/BAMOIjb.jpg

here’s the gradient map effect when the image is a PNG:

http://i.imgur.com/7oO7q3X.png

here’s the same effect when the image is a JPEG:

http://i.imgur.com/1bHmKdv.jpg

(there is a white and black blog on this image to show that the colours produced aren’t the same red and blue used in the settings).

I have ImageMagick installed on the server, so it is using that version of the plugin.

https://wordpress.org/plugins/twotonefx/