The exception is in the ultraviolet. At least until MAVEN's arrival, the ultraviolet capabilities of the fleet at Mars were very limited. Thus Hubble periodically checks in with the Space Telescope Imaging Spectrograph (STIS) and with the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC).
Below is the most recent view available from STIS, taken on April 26, 2012. Several observations have been taken this year but none have yet been released.
This year, ACS/SBC has also had some looks, some of which have been released. One dataset was obtained on May 30, 2014, for the purpose of preparing for MAVEN, providing a baseline for its observations to be compared with Hubble's past observations.
Hubble also took a look with its Wide Field Camera 3 (WFC3), the successor to its old workhorse, the Wide Field/Planetary Camera 2 (WFPC2). This was during the closest approach, when the orbiters were hiding behind Mars relative to the comet. The primary purpose was to provide a picture for posterity, covering closest approach, but given that many of the exposures overexposed much of the disk, I do wonder if they were trying to study the terminator region. However, Mars was quite distant during the comet flyby. As a result, its apparent diameter was less than six arc seconds, much farther away than it has been during most previous Hubble observations. For a comparison, I have put a WFPC2 image of Mars from the 2003 close approach next to an image from October 19, 2014. And, given that the pixel scale was somewhat coarser on WFPC2 compared to WFC3, the actual apparent size different is slightly greater.
A rough RGB process didn't offer much.
Processed Images Copyright Ted Stryk
Raw Hubble Data Courtesy NASA/STScI