Core: Extend Color API

* Fix issues with alpha value <> transparency
* Use type traits to convert between different color classes (fixes #14515)

src/Mod/Part/Gui/ViewProvider.cpp

@@ -88,7 +88,7 @@ void ViewProviderPart::applyTransparency(float transparency, std::vector<App::Co
         for (auto& j : colors) {
             // transparency hasn't been set for this face
             if (j.a == 0.0) {
-                j.a = transparency/100.0F;  // transparency comes in percent
+                j.setTransparency(transparency/100.0F);  // transparency comes in percent
             }
         }
     }