The intro music in the C64 version was originally written for Outrun Europe, an Outrun sequel developed by Probe, which was shelved when the deal to convert Turbo Outrun form the arcades was agreed. After Turbo was released, Outrun Europe was deemed not to be good enough to release, and recoded from scratch, ultimately released as Outrun Europa.
Turbo Outrun is very well programmed, but there are things that a 4.77MHz Tandy 1000 simply cannot do, including full-frame 16-color screen memory updates with any semblance of speed. To offer Tandy owners another option, the developers added an additional graphics option: Tandy 4-color. Selecting this runs the game in CGA. :-)