When looking at text on screen, colours of individual letters change periodically - from black to navy blue. Not computer; overworking eyes? Diabetes?

Macular issue. The variations in the colors of the letters localizes the problem area to the central retina, the macula lutea, aka the fovea centralis. Diabetes is not impossible but not likely without a history. A retinal toxicity to the antibiotics for h. Pylori is also possible. A retina specialist or general ophthalmologist is the starting point for a dilated examination of the back of your eyes asap.