Dyslexia is a term used to refer to a number of complex specific learning difficulties that often also include problems with reading and spelling.
Dyslexia is really about information processing: dyslexic people may have difficulty processing and remembering information they see and hear. This can affect learning and the acquisition of literacy skills. British Dyslexia Association
Visual Stress often runs concurrently with dyslexia, and it has been established that there is a far greater propensity amongst the dyslexic population to suffer from Visual Stress.
It is sensible to ensure that anyone who struggles to read or who shows or reports visual discomfort when looking at a book, should be offered testing with a coloured overlay.