It is not happening to all of my students, but at least three have shown me that when they click the color in Set Pen Color, it opens up a blank web page.
I already reported this, see
tl;dr:
it's an issue with chrome on MacOS that Snap! can't actually control.
It "used to be" 
a problem with the GPU in the browser. You may check if turning off hardware acceleration solves your problem in the short term.