Human interface guide: what are the most recommended colors for the user interface of a mobile application that is used in sunlight?

To use the application, users have to increase the brightness of the screen and this causes the battery to run out faster and the best experience is not yet perceived.

It is an application that users use daily for 6 hours under sunlight to take meter readings located in customers' homes.

They take a tour of the street visiting around 500 houses.