colour of these slopes there, what is that if not grass ?
Very interesting question, the geology of the area is very complex. The black terrain that you see also on many others of the serie is made of chippings of schist, with outcrops of quartz here and there. Here further on in this photo you get some limestone rocks (white outcrop in the foreground). The red stones are also a sort of limestone-sandstone impregrated of iron (from where is the colour), this stone appears in many places of the Pyrenees and run along the range like a fault. This stone is very appreciated for its capacities of sharpening knifes. On the west slope of the peak Fulsa in the background (right) these red stones surprisingly give place to granitic slopes (surprisingly because most of the Suelsa-Fulsa range is made of limestone).
Here you are :)