The Quarian response to radiation of any kind is likely the reason they are fully covered and inclined to block light radiation on their face as well. Their immune response is likely best served by reducing the amount of light exposure they get and would block as much light as necessary by their particular affliction; some might want more, others less.
As you stated in the question profile, the Quarians have a weakened immune system. Since we don't have any Quarians for analysis, we will have to use our own physiology as a reference. A weak immune system might make them sensitive to particular wavelengths of radiation. There are several immune conditions in humans that are affected by visible light such as lupus. There are, of course, others, this is being used as and example of an immune disorder and photo-sensitivity.
Symptoms vary from person to person, and may come and go. Almost
everyone with SLE has joint pain and swelling. Some develop arthritis.
Frequently affected joints are the fingers, hands, wrists, and knees.
- Chest pain when taking a deep breath
- Fever with no other cause
- General discomfort, uneasiness, or ill feeling (malaise)
- Hair loss
- Mouth sores
- Sensitivity to sunlight
- Skin rash -- a "butterfly" rash over the cheeks and bridge of the nose affects about half of people with SLE. The rash gets worse
in sunlight. The rash may also be widespread.
- Swollen lymph nodes
I am not supposing that every Quarian has lupus. I am supposing that a race with immunity and environmental sensitivity may need to protect themselves from even the smallest doses of radiation unless they are in an environment tailored to their particular sensitivities.
Since they are artistically inclined, they may possess a symbolic means of identification that allows them to recognize each other even if their visors are occluded in hostile environments.