The color was THE major reason why the scientists studying the Pathfinder and Viking sites concluded these deposits were carbonate or sulfate.
The rocks were both hard and white. There are alot of hard rocks obviously. But few that are both hard and white.
If the Sojourner rover just travelled over a hard rock it would not have been interesting. It was because these very hard rocks were white that led to the theory on their origin.
Color was a MAJOR part of their deductions about these rocks.
Let me put is this way: if the rocks were not white they would not have deduced they were carbonate or sulfate.

Bob Clark