No individual colour making up this wave travels faster than the grid lines, but the sum of all components appears to outrace the grid. This is how “group velocities” can exceed the speed of light without carrying information.
http://www.gregegan.net/APPLETS/20/20.html">https://www.gregegan.net/APPLETS/2...

If the refractive index falls rapidly enough as the frequency of the light increases, the group velocity can become negative. This has been achieved in real experiments (for a narrow frequency band): http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html">https://www.nature.com/nature/jo...