The article does not attempt to "prove" there exits life on Mars as you assume I was attempting to do.
A key fact is that this lead member of the Mars Express team believes the colors detected by their imager are real and represents actual colors on the Martian surface.
Once you know these colors exist then you can investigate various hypotheses for their origin. Because Mars Express also detected free water at the surface during the southern polar Summer, this member of the team believes the possibility that color changes are due to life is still an open possibility.
An interesting question is how warm the temperatures got at these near polar sites where they deteced surface water. TES on Mars Global Surveyor showed temperatures could exceed 0 C further than 70 south latitude during southern Summer. Were these some sites that also had free water?

Bob Clark