I need some graphic design advice on finding a quotable piece of information regarding the use of "highlighting" text on signage, which I hope you can help me with.

I'm in a dispute with my local council about a parking sign where they have put some vital information about the charging period which the have "highlighted" on a sign with a generally white background by using black text on a grey background. Common sense tells me that this low contrast combination of black text on a grey background is not a "highlighting" strategy, but rather obscures the text instead. Unfortunately my local council has no common sense, so I need to justify my "crackpot" theory with a reference to some respected source, such as a typographical text book or learned paper.

Can anyone point me in the direction of such a reference I could quote in my defence? Many thanks in anticipation of your support.