Fernando is one of the many colorblind individuals who has difficulty distinguishing between red and green objects (there are other kinds of colorblindness, too). Given that the whole point of this interactive is to see where the curve is red and where it's green, a colorblind user would have a hard time understanding what this interactive is supposed to do.
The last thing we want is for any of our users to be confused about anything we try to explain in our books. We quickly went to work on a version that would work for colorblind users, and our first stab at the problem resulted in this:
While this does indeed give colorblind users the ability to see what we were trying to explain in this interactive, we found that this could lead to confusion in some other areas. While testing this change, we realized that the empty spaces between red dashes could lead some people to believe that they were manipulating a discontinuous function -- and it doesn't help that the section on concavity comes right after the section that talks about continuity in Hands-On Precalculus.
We settled on the following hybrid solution, which gets the point across while keeping the function continuous and colorblind-friendly:
You can read more about Fernando's journey through the world of calculus on his blog. Thanks to his feedback, version 1.3 of Hands-On Precalculus and Hands-On Calculus will have interactives that are more friendly to colorblind users, plus numerous other improvements. The update is scheduled to be released within the next couple of weeks, so stay tuned!