The ruling on wearing one's adornment on the Day of Aashooraa
Question

I am a student in the girls college and there is a huge number of shiah living among us. Currently they are wearing black for the occasion of Aashooraa . Is it permissible for us in response to wear bright colours and extra adornment for the sole purpose of annoying them?! Is it permissible for us to backbite about them and pray against them, knowing that they have clearly expressed hatred for us? And I have seen one of them wearing amulets on which are written mysterious words and she had a stick in her hand with which she was pointing to one of the students, and I was harmed by that and am still being harmed. May Allaah reward you with good.
Praise be to Allah.It is not permissible for you to wear fancy clothes or anything else on Aashooraa because that may make ignorant people or those with ulterior motives think that the Sunnis are happy about the killing of al-Husayn ibn Ali (may Allaah be pleased with them both), Allaah forbid that that happened with the approval of the Sunnis. 
With regard to your interactions with them by backbiting about them and praying against them, and other kinds of actions which are indicative of hatred, this serves no purpose. What we have to do is to strive to call them (to Islam) and to influence them and reform them. If a person is unable to do that, then he or she should turn away from them and leave the dawah activities for those who are able to do them, and not do anything that may put obstacles in the path of dawah.  
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said:  
Because of the killing of al-Husayn, the Shaytaan started to spread two kinds of bidah (innovation) amongst the people: the bidah of grieving and wailing on the Day of Aashooraa, striking the cheeks, screaming, weeping and reciting eulogies ; and the bidah of expressing happiness and joy. So some express grief and others express joy, so they started to like the idea of wearing kohl, taking a bath, spending on their families and preparing special kinds of food on the day of Aashooraa and every bidah is a going astray; none of the four imams of the Muslims or others approved of either of these things (either expressing grief or expressing joy) 
(Minhaaj al-Sunnah, 4/554-556).