We share the same exact sentiment with Kevin of Guide Studios on this.
Notwithstanding having a good wayfinding system, which is IMPERATIVE in an indoor car park, graphic treatments need to be bright, clear & welcoming at the same time. Why? Not least, to give visitors & those who frequent the parking area a sense of safety & security. Apart from signage & gateway environmental graphics treatments at the entrances & on the building, car park treatments provide people with the first impressions of the place they are at.
Would you rather park at a dark, gloomy, dodgy looking car park or one that is well-signed, brightly lit & with great graphics?
We at Fusion Werks will always always always, strongly recommend focusing 'feel good' graphic design to be invested on & applied to, car park areas.