The theme for this tile set is the pipe organ stop tablets.  However, it is not that general, but rather using the style stop tablets used on that magnificent organ of past movie house days, the Wurlitzer Theatre Organ.  While the tablets on the classic organ are generally a single color, usually white, the Theatre Organ tabs were of several colors to assist in finding certain types of sounds.  They were as follows: 
                  RED - reeds 
                  YELLOW - strings             
                  GREEN - percussion
                  BLACK - couplers and percussion
                  WHITE - flute and foundation
Remember that the Theatre Organ contained in the pipe chambers actual percussion instruments such as various drums, and even an actual piano.  The above colors varied slightly with manufacturers, and varies on modern electronic imitations. May you hear in your imagination the magnificent sound of "the Mighty Wurlitzer" as you use these tiles!!                 
                G.D.V.