Goes back to what I was saying about DDA in Tyresmokes post.
White text on a black background could be best for a person with visual impairment, but inaccessible for someone with dyslexia.source:http://www.dyslexia-unlocked.com/accessibility.html
edit: Completely forgot to mention the fact dyslexia is a protected characteristic under the DDA/Equality Act - hence my surprise it isn't specifically included in the PSV legislation.
If legislation is in place to ensure individuals aren't discriminated against, then it is a surprise that the most difficult colours a dyslexic person is likely to encounter, is seemingly seeing an increase in popularity amongst bus operators.