Haemoglobin is a red-coloured pigment. If you are iron deficient, there may be less haemoglobin in your blood and you may look pale. For some, this is most notable on their face, nails, inner mouth and the lining of the eyes.
If you pull your lower eyelid down, the colour of the lining should be a rich, red colour. If it is a very pale peach colour or yellow, this may indicate that you have iron deficiency or iron deficiency anaemia.