Yes, fire was the first thing that came to my mind when I saw it. From the nearby road the first impression is of a charred log or post sticking out of the ground.
Yet, there was absolutely no signs of any charring or burning surrounding the site either on the ground or anywhere near the stone. Rubbing my hands over the stone there were no visible soot/smoke traces that came off.

My most recent theory is that due to its position on the machair, where the stone is continually sandblasted in all directions, that this sandblasting is bringing out the hidden natural pigmentation of the rock (a bit like polishing a piece of agate found in a stream) and producing very pronounced colouration in both the black and the peach coloured one.

FTC