Add a new layer in the layer panel, drag it down. If you want to have the color on every page, make a rectangle on the master with the color you need and extend it up to the bleed frame.For each such color make another layer and a frame.
Export to PDF with layers and no flattening.
For dyslexic readers, I would think you would want to make the copy to be as easy to read as possible, here is a link to some general readability tips:
The selection of a font would also be important, perhaps there is some research on the topic of best fonts for dyslexics.
For general InDesign help, there are many good tutorials on youTube and Lynda.com
Come back here to search for specific answers to your questions, it's probably already been answered.
BTW, what exactly do you mean by accessible PDFs?
You may want to take a look at this typeface designed for dyslexics -- Dyslexie font: optimal reading comfort - Dyslexie Font By making sure that certain characters are NOT mirror images of others, the designer made it easier to read for dyslexics.