Fixes: U4-4557 Color Picker - default is black, but it can't be added without choosing another color first

Sets color to be black by default, so will be added if you click add
without setting anything, so its consistent with the default swatch

src/Umbraco.Web.UI.Client/src/views/propertyeditors/colorpicker/multicolorpicker.controller.js

@@ -1,8 +1,9 @@
 angular.module("umbraco").controller("Umbraco.PrevalueEditors.MultiColorPickerController",
     function ($scope, $timeout, assetsService, angularHelper, $element) {
         //NOTE: We need to make each color an object, not just a string because you cannot 2-way bind to a primitive.
-
-        $scope.newColor = "";
+        var defaultColor = "000000";
+        
+        $scope.newColor = defaultColor;
         $scope.hasError = false;
 
         assetsService.load([
@@ -67,7 +68,7 @@
                 });
                 if (!exists) {
                     $scope.model.value.push({ value: $scope.newColor });
-                    $scope.newColor = "";
+                    $scope.newColor = defaultColor;
                     $scope.hasError = false;
                     return;
                 }