After halachic midday on Tish'a B'Av, you may do any activities except the five activities forbidden on Tish'a B'Av (washing, anointing, eating/drinking, marital intercourse, wearing leather shoes) or the Nine Days.  But you may not greet anyone, or reply to someone else's greeting to you, including saying “hello,” “good morning,” “how are you,” etc., until after dark.