Ayyappa Devotees are seen wearing black coloured dress during the Ayyappa Deeksha period. This is the important ritual for Ayyappa Devotees who are in the early years of taking the deeksha.
However people who have taken the diksha for many years contineously would wear saffron coloured dress. There is an important reason behind this ritual of wearing black dress.
Black is said to be the colour which will absorbe all other colours. It also indicates a detached mind the one that is withdrawn from worldly activities. This helps the devotees focus on the deeksha and they would not get distracted with worldly activities.
Also the Ayyappa Deeksha is a communal activity which is taken in groups usually, Hence it helps bonding among the group members when they have a uniform dress code.
Apart from these generic reasons, the most important one is the relation between Lord Ayyappa and Shani. It is believed that one would get relief from Shani dosha by taking the Ayyappa Deeksha. This is because Lord Ayyappa commanded Shani not to trouble his devotees. Normally Shani affects a person for seven years, but Lord Ayyappa said to Shani that his devotees would take the deeksha for 41 days and experience the same difficulties that Shani would have given in a span of seven years by not caring for their outlook, eating only once in a day, bathing in cold water and sleeping on a floor and practicing brahmacharya. Thus Shani agreed that he would spare Ayyappa devotees from his cruel eyes.
In return Lord Ayyappa assured that all his devotees would wear black dress which is the colour associated with Lord Shani. This is the reason why lord Ayyappa devotees wear black dress.