Visually impaired conditions may include partially sighted, color blind, ...
A list of guidelines is currently being developed. To help you in the meantime, here are a number of links which you may find useful.
Firelily effective color contrast by Lighthouse International (list of publications).
Vischeck has a tool for simulating various vision deficiencies on either images or web pages. It's very slow, does only a so-so job, but the results are well worth looking at and may surprise you. Worth trying out your slides on.
Webexhibits How do things look to color blind people