People with open-angle glaucoma sometimes see halos or rainbows around bright lights (e.g. headlights at night), but not always. Glaucoma results in elevated intraocular pressure and thence pressure on the optic nerve, so it doesn't surprise me that there'd be visual effects -- but why halos and rainbows specifically and not, say, blurry vision or flashes of light or some other deviation from normal vision? And what factors govern when it happens, since it's not a constant state? Is that just caused by minor fluctuations in the pressure level, which I understand varies through the course of the day, or is there some other factor?
The so-called halos are typical for spikes of intraocular pressure rise in high-tension glaucoma (or ocular hypertension) forms, most often in melanin-dispersion syndrome (MDS, or pigment-dispersion syndrome) which is a trigger for glaucoma in myopes and often begins in early adulthood.
The halos appear when a spike in intraocular pressure leads to intracorneal edema, as the corneal endothelial cells which permanently pump the nutritious water out of the cornea cannot compete with the high pressure pushing the fluid into the cornea anymore. Thus, the cornea fills with water, leading to temporary loss of visual acuity and, as an early symptom, to halos and rainbows around sources of light due to increased dispersion of light. The regularity of the extracellular matrix, which is crucial for corneal transparency, is disturbed by the intrusion of water leading to cell displacement.
Basically, this can happen in other forms of high-tension glaucoma or ocular hypertension with momentarily raised intraocular pressure (IOP), too. Note that it only does in forms where the IOP rises quickly, whereas in a slow and constant rise (as it happens in many forms of chronic glaucoma) the endothelial cells can adapt and still drain the cornea from the excess water. So basically, the same happens in an attack of angle-closure glaucoma, just with the difference being that angle-closure glaucomas often don't go away without medical treatment, whereas in early adulthood IOP spikes due to MDS the halos often are the only symptom which is often triggered by exercise or in dim ambience light situations. This is due to the fact that when the iris moves (as in widened pupils due to adrenaline reactions when performing sports or whe the lights are dim) in a patient with MDS, it rubs against the lens, loses pigment by erosion and this then clogs the eye's drain (in the anterior chamber angle).
This can be seen as basic knowledge in ophthalmology. A first report on the mechanism leading to IOP rise dates back to 1953. The induction of IOP spikes by exercise has also been known for a longer period of time.
EDIT: As your question explicitly relates to open-angle glaucoma, I'd like to add that there are different types of open-angle glaucoma, and only some of them are known for IOP spikes that could lead to said halos and rainbows. Primary open-angle glaucoma usually doesn't, but other than the mentioned pigmentaray dispension syndrome, PEX glaucoma could, and herpes simplex trabeculitis is also not uncommon.
The American Academy of Ophthalmology has a good description on pigmentary glaucoma in their EyeWiki.
It can be also found in Clinical Ophthalmology: A Systematic Approach, 7th Edition. Jack J. Kanski, B, Bowling. 2011, Saunders, ISBN 978-0-7020-4093-1, p:365ff., and extensively in Basic and Clinical Science Course 2007-2008 Section 10: Glaucoma. American Academy of Ophthalmology, 2007, San Francisco, ISBN 978-0-7020-4093-1, p.101-103.