What is the difference between an "old work" junction box and a "new work" junction box? Is a "new work" junction box only for use in a house that is of new construction? Is there some reason a "new work" box can't be used to add a junction box in an old home? I saw junction boxes at my local home store labeled "new work" and "old work" and I want to know if I must use an "old work" one in my old (c. 1929) house. I need to install a round junction box in my bathroom in order to install a flush mount ceiling light fixture. The old fixture was a recessed florescent strip light that has been removed.

the colors on the wires coming out of the ceiling are as follows white(maybe gray) purple, green.yellow and red. I am attempting to install a ceiling fan and im clueless as to what wires attach to what..! The fans wire colors are blue white black and green HELP!!