Hello!
The shader from AUM is sensitive to changes of the scripted sun_light.
There´s a difference between the scripted sun_light (which is basically a dynamic, directional) light and these "sun"-settings in WED.
Changes in WED Map properties do mostly affect levelgeometry and models (their ambient gets affected by the static lightmap).
The scripted sun_light affects parts of models (and also your terrain).
You can dynamically change the color of the scripted sun_light by setting the sun_color vector. This however will probably not affect this particular shader.
A multitexture shader, that is sensitive to both sun_light and sun_color can be found in the templates (mtlFX.wdl). But you can only use three tiled textures and the shadowmap must be integrated in the blue channel of the RGB colormap.
You could try and change the code from the AUM shader to make it sensitive for sun_color (which has to be changed by script then, not in WED). Maybe you´ll find some inspiration in the template shader. Or you can simply use this shader from the templates, instead of the one from AUM ...
