Doors with glass panels – even bathrooms can have a frosted glass door; or use shutters, instead of solid wood panelling.
Doors that concertina are also a good idea if they are spaces that aren’t used frequently. Hallways often have no lights of their own so use colours that reflect light on the walls and doors with a tone a few shades lighter on the ceiling. Good colours to try are pastel hues: pale pink, teal and very light grey are very on trend right now. White can work but choose a warm white, not a bright white, as it can look too office-like in a home. (A hint of yellow or pink to it).
Most paint companies now have ranges specifically designed to reflect light, so look to these first. A collection of antique mirrors can also create points of light and interest and when placed on opposing walls in long spaces, or as a group opposite a window, can make a room look bigger.