At what point in the history of Superboy, he changed his colorful costume, starting to wear a black shirt with the logo and a pair of jeans? In which comic refers to the change?

ive also wondered about this. didnt start reading any of his stuff till way after and like 1 comic when he was still rockin leather.

This is what I researched:

July, 2002: Superboy was taken by Superman to stay with his parents in Smallville, which he did for some time (Superboy#100).

May, 2003: Finished the Young Justice Series (Young Justice #55)

July/August, 2003: In graduation day, Superboy appears even the colorful costume, without the leather jacket.In this series witnesses the death of Donna Troy. (Graduation Day).

September, 2003: One month after, based in Smallville with a new civilian identity as Clark Kent's cousin "Conner Kent", Superboy is asked by Superman to accept Cyborg's invitation to join a new incarnation of the Teen Titans. Here he is already using the black shirt with red symbol. Possibly a sign of mourning for the death of Donna Troy (However, there is no reference on that). (Teen Titans #1)