If the newly purchased cloth is made of silk, one can wear it as it is. If it is made of cotton, one must smear a little turmeric powder, wash and dry it before being worn; this practice is followed by elders as can be seen during Avani Avittam, especially by those elders who have the habit of purchasing new clothes for Avani Avittam.
During auspicious festivals like Deepavali one can wear new cloth (without washing) but it is not considered Madi [however it is not considered impure(theetu)]. Also one cannot do Thiruvaradhanam wearing new cloth without washing them as it is not Madi.