key_color_image

It is a command line application to which you can pass a color, and the application transforms the input images to that color scale image (as if it was a gray scale, but instead of gray, that color).
In addition, the application allows you to set the global alpha of the image, and translate a particular color to a totally transparent alpha (alpha = 0).

You can also disable alpha component, just in case the input image is a .jpg, or there is any restriction that makes the output image from being saved successfully.

If you do not know the color code, you can pass a color: -color input and the application will let you move the mouse on the screen, and when you press return, the color of the mouse pointer will be picked.
(This does not work properly in certain environments, but you can always move the mouse over your color before launching the application).