Discussion about this is taking place on the Portal, too, and the relevant passage has been posted there:
"MOELFRE stones Dwygyfylchi SH 721746
Three standing stones, 'red, white and blue', formerly here, were three women turned to stone for winnowing corn on a Sabbath morning; the colour of each stone was that of each woman's dress. Wynn c 1640/1906, 26-7; Sikes 1880, 376; Lowe 1912, 54-5"