It is difficult to define what is really meant by this. The closest definition would range from an uncomfortable sensation in or around the eye to positive low pain in the globe. Common cause are:
(a) Any error of refraction like myopia, hypermetropia or astigmatism. It is notable that the small errors cause aches, not large errors. With a large error the eye makes no attempt to compensate by exerting itself and therefore "comfortably" decides not to see. With a small error, it attempts to bring the object into focus continually and therefore strains, resulting in eye aches.
(b) Low grade inflammation in the eye, like conjunctivitis or iritis.
(d) Sinus problems.
(e) Uncorrected muscle problems in which there is a mild weakness. The attempt to maintain the parallel position causes continuous contraction and aches.
(f) Excess application of the eyes to near work or reading leading to passive congestion.
(g) Sometimes weak health with a poor accommodation capacity.
Coloured Vision (Chromatopsia)
It can be extremely unnerving to get up in the morning with coloured vision. There are various causes of the vision getting coloured. However, mostly they are of a short duration and usually a treatment of the causative condition or a discontinuation of some drug is sufficient. The following are only examples to show how even frequently-used drugs can cause this remarkable change.
Barbiturates (sleeping drugs): yellow vision
Streptomycin (antibiotic): yellow vision
Griseofulvin (anti fungal drug): green vision
Common diseases like jaundice cause yellow vision. Following a cataract operation or even an iridectomy, red or blue vision is common and gradually diminishes.