Client: So the website you have built with a black background and white text...
Me: As requested, yes...?
Client: Well we think it might be hard for blind people to read.
Me (stifled laughter):I see...what do you propose?
Client: To put up a sister site with white background and black text.
Me: for the benefit of the blind?
Me: OK then, it will obviously be billed as a whole new website and you understand that your website maintenance bills will be doubled each month?
Client: Yes, yes that's fine.
Subsequently the second website (for the blind!) was built and I charge them double each month than I did before. Result.