We have not specified the material and construction of the arena in the rules intentionally. This is because different material is available in different regions.
And for example at a school or regional competition they may not have the money to build arenas in aluminium or plexiglass. They just need something to form a maze with.
In addition a real rescue robot may not know exact how the building is made, not the materials nor the colors.
We in the TC writes the rules, but it is the local organizers in Japan that manufacture and prepare all material for the OC to run the competition.
Having this said I can tell you from my experience of many international events that they arenas will be made of wood painted white or close to white. The squares in maze should be 30cm, but some organizers count this as a CC measurement and have walls that is 2-3cm so the actual space between two walls is closer to 28cm, according to rule 1.1.3:
"Their width can reduce the available space for doorways and paths"
The height of the walls will most probably be 15cm to make it easier for the audience to see the robots.
// Fredrik Löfgren, OC and TC member 2016 & 2017