Switch (back) to our own Color type instead of using wxColour.

The reason is that wxColour's default constructor creates an invalid color (what is that even?). It is nicer to just have default be transparent.

src/script/value.cpp

@@ -341,11 +341,11 @@ public:
     }
   }
   Color toColor() const override {
-    Color c = parse_color(value);
-    if (!c.Ok()) {
+    optional<Color> c = parse_color(value);
+    if (!c.has_value()) {
       throw ScriptErrorConversion(value, typeName(), _TYPE_("color"));
     }
-    return c;
+    return *c;
   }
   wxDateTime toDateTime() const override {
     wxDateTime date;