I would have painted the whole underside purple.
Funny you mention that. Before having the car painted I took a mini poll of friends and potential buyers. It seems it was unanimous that if you're selling a car in the $50K - $100K range, it better be restored correctly.
The last car I sold to a guy in Germany, his only complaint was that the underside was painted completely.
Unfortunately when a "buyer" tells you that, you're best to listen.
The body shop would have preferred to paint everything purple because it would have been easier for them.