Append String and Variable to SQLite Column

I am trying to append a string value and a variable to a column in my database. Currently I have the following:

using (SQLiteCommand cmd = conn.CreateCommand())
{
    cmd.CommandText = "UPDATE Movies SET part = part || @part WHERE id = @id";
    cmd.Parameters.Add(new SQLiteParameter("@part", part));
    cmd.Parameters.Add(new SQLiteParameter("@id", id));
    conn.Open();
}

However, I would like to add a : delimiter between the old and new values.

Eg: The current value of the column part is 1. The value I want to append is 2. So the final result in the column should be 1:2.

I have tried:

cmd.CommandText = "UPDATE Movies SET part = part || ':' + @part WHERE id = @id";

But this seems to add 1 and 2 together.

Would anyone know the correct syntax for this?