This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I can confirm having just tried it with JAVA 1.6 that you need to use getContentPane().setBackground(Color.red);
Normaly with this version of JAVA the compiler will imply in some cases that you mean to refer to the property of the ContentPane (a JPanel instance) rather than the JFrame itself, to provide a sort of shortcut to the programer.
However as you've just seen this isn't a good habbit to get into because it doesn't work with every method of a JPanel as you have illistrated and your code will not work if compiled on older versions of JAVA in anycase.
Just to add it's also not a good idea because it causes confusion over what object's method you are really calling as well.