Thanks everybody for suggestions.
Here is a (temporary) page with pictures. First with no filters, subsequent filters at 0 (min), 30, 60, 90 (max):http://www.terragalleria.com/tmp/crosspol.html
By the way, the sand is from the Mesquite Dunes in Death Valley. More colors than expected !
I know it would have been nice to stick a bit of WB card, but the one I have at hand is too big and would not fit within my set-up.
You can see that the color gets warmer with the amount of polarization. I am still wondering why this is happening.
With polarizing filters parallel it is a bit too cold (and has more surface reflections than without filters !) and then becomes too warm at 90 degrees.