Herbie The Love Bug was a 1963 Volkswagen Beetle deluxe ragtop sedan painted in Volkswagen L87 pearl white. Under normal circumstances, the interior would be a matching white. However, Herbie's interior was painted a special non-reflective grey color so the camera and studio lights would not reflect.
The gas cap used in the film was fake. As every Herbie fan knows, Herbie's gas tank is under the hood, (as revealed plenty of times in the original movie). The fake gas tank was added so that the gem thieves could easily stash the diamond inside Herbie, without looking conspicuous. The tank was removed in the remaining sequels.
At the time of the movie release, Baskin and Robbins did a promotional ice cream flavor tie in called "Monte Carlo Crunch". The author does not remember the specific flavor and makeup of the ice cream used for the flavor.