All beige or similarly coloured crystalline rocks have those colours for very good geological and mineralogical reasons. They contain small amounts of hydrated iron oxide (usually within the feldspar) which ranges in colour from earthy yellow through to brown. The reason for its presence is due to a remobilization of very small amounts of iron-bearing fluids through the granite by late-stage or post-emplacement hydrothermal activity. The majority of the dissolved/remobilized compounds are derived from iron-bearing trioctahedral micas (biotite, annite) and to a lesser extent iron sulphide and iron oxide.
Because of the presence of the hydrated iron oxide the beige and yellowish-coloured granites cannot be exfoliated (flamed) without changing the state of crystallization of these poorly formed hydrated minerals. With the heat of exfoliation exceeding 1000 degrees C haematite is formed in small crystals and globules from the hydrated iron oxide and this changes the colour to pinkish.
Because of the elevated rate of water absorption (around 0.4%) and their unusual chemistries these granites need to be treated in ways different to most other granites. They are not meant to be cleaned with acid nor saturated with water containing alkaline compounds. Such chemicals will “burn” this type of stone. Expert advice should be sought on the use of these specific stone varieties.