This thread discussed the clouds seen from the rovers:
Clouds On Mars.
http://mer.rlproject.com/index.php?showtopic=476&hl=
Are there any multi-filter PANCAM images of the clouds?
They might be used to calibrate the color combinations working on the assumption the clouds should be white.
cf.,
From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Mars Express image of Valles Marineris shows green!
Newsgroups: sci.astro, alt.sci.planetary, alt.astronomy, sci.astro.amateur, sci.bio.misc
Date: 2004-01-24 04:20:40 PST
http://groups.google.co.uk/groups?th=8ea6ade76ffbdae0&
Bob Clark