Hello Ashish,
Just do one thing to confirm. Go into HTML5 mode, use Adaptation tool and set the field you want to be mandatory as mandatory. Publish this change to field property and test.
Whenever the field that is set as mandatory using KUT/Adaptation tool and not filled in before saving, you get this generic form of message:
Save rejected because field (<FieldName>) is defined as mandatory by key user.
Fieldname is automatically picked up while displaying this message. You do not have to do any coding to achieve this. Just set it as mandatory using KUT and you would get this message with the Red color that you expect. I had mentioned this even before.
Regards,
Chandan