As you can see your image is connected to the *density shader* input, so it only affects density of the clouds. There is of course no "colour shader" input for clouds at this time so you can't do what you would like to do just yet. There should be a method to achieve this in the future, whether through a dedicated "Cloud colour" input or a generic ability to "drive" any setting with a shader (most ideal).
Do note though that even if you could get this to work using an image like that to color clouds wouldn't really look very realistic. You could achieve a better result simply by adjusting the built-in color controls or creating an atmosphere that lit the clouds in that way (heavy red decay for example).
- Oshyan
