Since Mountain Lion, the highlight tool in Preview allows one to select among a small number of colors. Is there a way to customize the colors (add to or change)?
What they are talking about is having more color choices for highlighting text than are available by default. Both the Yosemite choices in your post and the Mavericks choices (see below) limit the user to 5 preset colors. There doesn't appear to be any way to use the OS-built-in Apple color picker to choose anything other than this very limited set. Very frustrating.
When you click the colour drop down on the annotations toolbar, you should see an Other option, that will allow you to pick arbitrary colours.
I don't know what you guys really talking about, but in 10.10.2 everything works just through the right click: