I'll grant that trying to judge much from pancam spectra, let alone from our own non-calibrated color comparisons can be tough. My observation about Bounce isn't anything so subtle, though.

In terms of overall albedo, the basaltic rocks at Gusev were much, much darker than the dust that covered them.

In contrast, Bounce is much, much brighter than the dust. Even if the Meridiani dust is darker than the Gusev dust, I don't think that difference alone is enough to explain the apperance. The exposed RAT area on Bounce is much brighter than the basalt at Gusev, and if anything, even looks brighter than the outcrop in Eagle crater (and the Eagle outcrop produced red-brown dust from grinding, not white dust).

Certainly, there are light colored igneous rocks, but syenite or rhyolite would be easily distinguishable from basalt by the APXS -- lots more silicon, less iron.

I'm not making any wild claims that it has to be limestone or has fossils or that NASA is covering something up. I'm just curious why what is supposedly an unremarkable piece of basalt is so much lighter than other rocks of that composition.