For some reason best known to me I had used blue blocks for various lessons with white text. To make life simple I put hyperlinks on the text so that clicking a lesson would take me to the appropriate register. Of course, the text turns blue and gets underlined and is virtually invisible. Or it goes purple as a 'visited' link.

Ok, that'll be easy to fix, I say to myself. Uh-uh. Tried almost every menu. Tried the Microsoft help files but I seldom get much joy there anyway. Thought about the PCAdvisor forums but they don't have an obvious Excel place. Normally I would have popped into Woody's Lounge but that's under some redevelopment and temporarily off-line. Eventually I find Vastly Important Notes blog by Phil Libin and get the answer. Another addition to my favourites list.
You need Format | Styles and there you can change Hyperlink and Followed Hyperlink colours (or colors in MS speak).

That's better!