Can you reverse type onto coloured blocks? What precautions should be taken when doing so?
Which learning difficulties are we talking about here? Dyslexia or other? If dyslexia you should follow this thread: http://www.typophile.com/node/36906
In case of dyslexia I would not do so because of the way shapes and colour is perceived by a dyslexic.
Why do so in the first place? It reduces legibility and makes it harder even for those without difficulties.
I'm certainly no expert, but 'learning difficulties' is rather vague. We'd probably need to know more info as well...age target? Screen? Print? In what context?
I think reversed type is useful on-screen. I wouldn't do it a lot on paper, but that's not based on any specific accessibility guideline that I'm aware of.