Even in this day and age, it looks pretty good. Fulci knows how to create the right atmosphere of tension, and he creates it in a variety of ways. The disturbing music ('seven notes in the dark'), the semi-dark rooms, the play of light and shadow. Characters move from light to shadow and back again. And out of the seemingly total darkness there is always a light, be it the light at the end of a dark tunnel where the radio fades and hisses, or the light of headlights seen in the darkness in the back window of a cab, or a red light that somehow gets smaller and smaller. The answer to what this light is and why it is fading away is at the end of the movie.
And yet the main way to create suspense is with colors. Predominantly, bright red color, which is designed to excite our nerves. There is a lot of red here, sometimes it seems even too much. Red details of clothes, other objects, bright red room interiors (e.g. wallpaper and lampshade) - all these symbols are alarming and disturbing; like Virginia's visions, they are omens of near distress. Only unlike the visions, the color red is meant to evoke an underlying anxiety in the viewer.
Other colors include unnaturally yellow 'French' cigarettes, a blue ashtray, and purple in the clothes.
It is also worth noting the presence of many close-ups of faces and objects. This also creates a certain atmosphere, especially during the visions of the protagonist, when loud disturbing music plays, and the moment of what is happening in reality sharply changes with the vision, and then the camera shakes and 'runs over' Virginia's face.