Because that is not the real image but a composite to show you where that aurora is located. The aura's are in UV and you can't see that on a visual image.

The full-color disk of Jupiter in this image was separately photographed at a different time by Hubble's Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of the outer planets.

http://hubblesite.org/newscenter/archive/releases/2016/24

To highlight changes in the auroras, Hubble is observing Jupiter almost daily for several months. Using this series of far-ultraviolet images from Hubble's Space Telescope Imaging Spectrograph, it is possible for scientists to create videos that demonstrate the movement of the vivid auroras, which cover areas bigger than the Earth.

http://hubblesite.org/newscenter/archive/releases/2016/24/full/

This image is a composite of separate exposures acquired by the STIS and WFC3/UVIS instruments. Several filters were used to sample various wavelengths. The color results from assigning different hues (colors) to each monochromatic (grayscale) image associated with an individual filter. In this case, the assigned colors are:

http://hubblesite.org/newscenter/archive/releases/2016/24/fastfacts/