In honor of the first year anniversary of Assassination Classroom, the Shounen Jump is holding a contest. Fans will submit what they would expect the blackboard of Class E would look like. The deadline is the 29th of July and Matsui himself will be judging the contest. The winner will see Matsui use their entry for a center color in September.
I’m not sure if fans outside of Japan wouldn’t qualify, but there’s nothing in the rules about it (as far as I can tell)