Fix almende#3713 - Node color overriden by Group Color (almende#3870)

* Checks if color is not defined at node level before overriding it with the Group color

* Checking if newOptions is not undefined before checking for color property

lib/network/modules/components/Node.js

@@ -212,8 +212,12 @@ class Node {
     var groupObj = groupList.get(group);
 
     // Skip merging of group font options into parent; these are required to be distinct for labels
-    // TODO: It might not be a good idea either to merge the rest of the options, investigate this. 
-    util.selectiveNotDeepExtend(['font'], parentOptions, groupObj);
+    // Also skip mergin of color IF it is already defined in the node itself. This is to avoid the color of the
+    // group overriding the color set at the node level
+    // TODO: It might not be a good idea either to merge the rest of the options, investigate this.
+    var skipProperties = ['font'];
+    if (newOptions !== undefined && newOptions.color !== undefined && newOptions.color != null) skipProperties.push('color');
+    util.selectiveNotDeepExtend(skipProperties, parentOptions, groupObj);
 
     // the color object needs to be completely defined.
     // Since groups can partially overwrite the colors, we parse it again, just in case.