Simple is powerful. Paint your doors, trims, ceiling and cornices with gradients from the same colour family, and choose one main colour for the walls. Bedrooms can always be more playful, but go for minimum contrast, as it soothes the colour palette from seeming too jolting.
Colours with a white, grey or blue/green base will reflect light, preventing rooms from seeming too crowded by dark colours and patterns.
You need light in the centre of a room without relying on floor or table lamps. A few metal pieces will bounce some light around around and help alleviate all items from feeling the same.
Keep with the times. Modern apartments are minimal: trims, skirtings and ceilings are clean-cut and the palette is simple, spliced with some zests of colour. Period-inspired interiors are quite the opposite, often favouring ornate detailing. In a busier period-style composition, identify one defining feature (like panels or cornicing) and highlight it with bold colour. Invest in a particular interior style, and your apartment will appear a larger and more cohesive space.
There is a common misconception that apartment living needs to be treated as one big white box. Just adding a hint of colour to the walls can take the apartment from feeling clinical to full of life. Suggestions? Bring lush plants into your decor, try sh ideas by arranging flowers, or add a touch of your favourite childhood colours.