Should curtains match the wall colour or contrast with it?

Matching tones make a room read larger and calmer because the eye is not stopped at the window. Contrast turns the window into a feature. In a small room, match; in a room with an architectural window worth showing off, contrast.

How high should curtains be hung?

Higher than the window frame, almost always. Mounting the rod partway between the frame and the ceiling adds visual height, and on high ceilings taking it close to the ceiling line stops the window looking stranded.

What works on an arched or triangle window?

Custom-shaped shades follow the angle and keep the geometry visible; straight curtains hung above the whole opening hide it. Which is right depends on whether the shape is a feature you want to keep or a problem you want to cover.