Or, my observation of the role of colour contrast.
When we're given lined A4 papers for an exam, what's on the paper? (Lines, duh.)Ink and paper is serving a lot of functions in our world today. Not just exam papers and web pages, but many others: money, legal documents, academic certificates... more.
Before we turn on the power for our computers, what's on the screen? (And you wouldn't see this without shading it.)
Back when we were still kids, and the lights were turned off, what is it that we see? Or, do not see?
I'm just truly amazed by the effect of colour contrast in facilitating effective communication, even communication with self (diaries, blogs, lecture notes, etc). For my first PBL case last semester, I just write G6PDD. And it tells of a whole plethora of things from the symptoms such as lethargy and weakness, to its cause which is a single enzyme deficiency, as well as the mechanisms linking them together. We learned that 400 million people globally are affected by this X-linked, recessively inherited condition.
And the fact that I can tell which keys to press to produce this post (as we all can). I think it's immensely amazing. The human brain and eye has got some real super-duper processing power.
Otherwise we would probably click on emails in our Inbox that are really just spam and contain nothing sensible, and waste our living time doing so.