In the "sun path diagram" window on the right side, expand the "Shading Mask" heading. If you hold down the "Manage" button, a list will appear. Scroll down to "Customize Scale . . .". Once you have selected this you can manipulate the "value range" at the bottom to whatever scale you wish.
I'm not sure if this answers your question or not, because you mentioned already trying "customize scale".
If you want to change the shadow color in an overshadowing diagram you can change all shadow colors using "user preferences" or you can change the shadow color of individual zones, through the "zone properties . . . " under the zone management tab. (right click on the name of a zone in the zone management list and the "zone properties . . ." option will show up in a list.)
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