Elven Attunement Markings

After Elf children have attuned to their Elements, their bodies become receptive to the related magic. Just by perceiving the world around them, they start absorbing these energies and storing them in the attunement crystals on their spine. The current scientific consensus is that the constant exposure to magic causes mutations on the outer surfaces of the body.

Hair Color Shift

Air attunement tends to make the hair pale whereas Ground attunement darkens it. Water makes the colors more vibrant, leading to colors that other species might call "unnatural".
 
  Energy does not appear to affect it in any way, but there might simply be a lack of data. Most Elf bloodlines that are attuned to Energy originate from the desert, and their pre-attunement hair color tends to be pitch black. Therefore, shifts in the hue would be impossible to detect.

Iris Color Shift

Air attunement tends to brighten the iris and give it a cooler hue. For example, a child born with olive green eyes would end up with a bluish gray eye color. Ground attunement has the opposite effect, so brown eyes would turn into a dark red. Water attunement makes the color more vibrant, for example by turning brown eyes into purple or giving gray eyes a blue tint. Energy attunement darkens the eye color.
  The secondary elements - Fire, Lava, Plant, and Fog - also appear to shift the eye color in similar directions as the adjacent primary element. For example, Fire darkens the iris whereas Fog makes it more vibrant.
 

Pupil Glow

If a person is attuned to Energy or Fire, their pupils are likely to glow. This might also be observed in people with a sufficiently strong Lava attunement. As a rule of thumb, Energy and Lava tend to cause a red glow whereas Fire leads to an orange or yellow glow.

Patterns

The secondary elements manifest as patterns on the skin and possibly the hair.
 
  Fire causes red or orange stripes on skin and hair. Lava causes dark, hardened patches on the skin. Plant causes ruptures that reveal a green glow. Fog causes glittery stripes on the skin and gives the hair a sparkly appearance.