"The majority of the stones are grey, but Stones 54, 55, 101, and 156 exhibit an orange hue while Stones 53, 56, and 154 are purple-grey. " *
Out of interest I charted the PXRF data for Fe % for each stone from Nash et al 2020**
PXF penetrates a little way into the stone so doesn't just measure the crust but it seems there is only a weak correlation between the perceived colours and the Fe concentration of the surface layers. Which is a bit mysterious.
Maybe the colouration is shallower than the PXRF sampling or it isn't an Iron colour that is being seen. Any thoughts?
*David Field, Hugo Anderson-Whymark, Neil Linford, Martyn Barber, Mark Bowden, Paul Linford, Peter Topping, , Marcus Abbott, Paul Bryan, Deborah Cunliffe, Caroline Hardie, Louise Martin, Andy Payne, Trevor Pearson, Fiona Small, Nicky Smith, Sharon Soutar and Helen Winton (2015). Analytical Surveys of Stonehenge and its Environs, 2009–2013: Part 2 – the Stones. Proceedings of the Prehistoric Society, 81, pp 125-148 doi:10.1017/ppr.2015.2