A Concise History of the X-15 Research Airplane 

The top and bottom of the fuselage were usually covered in frost because the LOX tank was integral with the fuselage. Oxygen is liquid at -297 degrees Fahrenheit. All three X-15s normally carried a yellow NASA banner on their vertical stabilizers. (U.S. Air Force)

Jenkins, Dennis R., Hypersonics Before the Shuttle A Concise History of the X-15 Research Airplane. Published by NASA through the U.S. Government Printing Office, Washington D.C.  128 p. June 2000.