Here are the captions that accompany those JPL images of Mazatzal:

This image was taken by the Mars Exploration Rover Spirit's panoramic camera during the rover's grinding of the rock dubbed "Mazatzal" with its rock abrasion tool. The picture shows the rock after two targets dubbed "New York" (left) and "Illinois" were brushed on sol 81. The exposed, dark surface is a second coating beneath a top white veneer. This approximate true-color image was created using the panoramic camera's red, green and blue filters.
http://www.jpl.nasa.gov/mer2004/rover-images/apr-01-2004/captions/image-7.html

This image was taken by the Mars Exploration Rover Spirit's panoramic camera during the rover's grinding of the rock dubbed "Mazatzal" with its rock abrasion tool. The picture shows the rock after the rover drilled 3.8 millimeters (.15 inches) into the target dubbed "New York" on Sol 82. The dark grey coating seen after brushing remains on the right side of the hole, while the left side is the underlying basaltic rock. This approximate true-color image was created using the panoramic camera's red, green and blue filters.
http://www.jpl.nasa.gov/mer2004/rover-images/apr-01-2004/captions/image-8.html

Maybe by "grey" they mean "any dark color seen on Mars that is not red or brown."

Bob Clark