They don't have that plane anymore.
Back 15 or 20 yrs ago, Southwest was leasing some aircraft to give them faster capacity increases...Boeing could not keep pace with the demand Southwest had for new aircraft.
Certain leases were made with the precondition that Southwest not repaint them (except to paint the name someplace).
At one time or another Southwest has flown planes with the paint jobs of Frontier (old version), Aer Lingus, Air Florida, and several others.
They are not changing the paint scheme away from the red, orange, and gold. As much as people gripe about it today, it looked real good in 1971. By the same token...if a paint scheme can be a safety device...this one is. You have to admit their colors are incredibly visible against a blue (sky) backdrop. When one passes overhead with that red belly you know INSTANTLY whose plane it is.
I do not work at Southwest, but here is my guess as to their reasoning: You see airlines start to lose money or passenger traffic, they (and their ad agency) decide a makeover is needed to project a fresh image. Folks must not like the "old" airline so they attempt to become a new one. This seldom works but companies do it anyway.
Since Southwest hasn't lost money since they started earning a profit in '73...and traffic grows by roughly 10 to 15% every year....I suppose they figure it is better to roll their profits into new aircraft and growth rather than aircraft paint. They seem generally content with their own identity.
The whales and the flags are neat paint jobs, however.