Opportunity has a little probe on it that tells the mineralogy.
The colour doesnt matter in the slightest. Both of your conclusions are not valid.

The fact you got white rocks here doesn't for a millisecond mean they are the same as white rocks elsewhere.

There are a plethora of white rocks and minerals - not all are water related.

Quartzite is white.
Feldspars can be white or light pink.
So can Sodium Carbonate (and if it's that, then H2O bites the dust)

Keep an open mind until Opportunity has the opportunity to do its stuff.

BTW, as I posted above, the weathering of the bedrock looks like it could be kinda karstic. :-)