The dark red color of these rocks comes from iron oxide . However , not all the Old Red Sandstone is red or sandstone â '' the sequence also includes conglomerates , mudstones , siltstones and thin limestones and colors can range from gray and green through red to purple . The familiar red color of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone â '' the sequence also includes conglomerates , mudstones , siltstones and thin limestones and colors can range from gray and green through red to purple .