Use the Insert() function to put the colon in:
e.Row.Cells[i].Text = e.Row.Cells[i].Text.Insert(3,":");
Use the SubString() function to do the truncation:
e.Row.Cells[i].Text = e.Row.Cells[i].Text.Substring(1);
You can combine these two functions into a single statement:
e.Row.Cells[i].Text = e.Row.Cells[i].Text.Insert(2,":").Substring(1);