I'd suggest backgrounds of icons can have colors matching their purpose. Nowadays, it's hard to recognise from the icons what to use the stop for, tiny icons inside icons are the only indication for new users... If there's no general purpose, it could stay grey, and if icons have multiple purposes, they could be multicolor. Down below are some examples... Green for passengers, Yellow for Mail, Blue for Goods and Red for decorative items without any purpose. Hope it's useful and making Simutrans more convenient or at least an some inspiration..
PS: I'm colorblind so I tried to be considerate with color picking.