The 3 main constitutional types
There are 3 main Iris colour types, namely Brown, Blue and Grey. There is also an exception to these basic types known as a Biliary or Mixed Constitution, namely part blue and part brown. This type display a combination of factors that are found in both the blue and the brown eyed types.
Where the iris pigmentation is slight, the iris appears blue as is commonly found in the northern European type, where less pigmentation is required for protection against intense sunlight. On the other hand, with an increase in pigmentation, the colour becomes more grey and proceeds to brown and on to dark brown, as is found in the Mediterranean, Middle Eastern, African and Indian types. Over the generations, interbreeding has, in some instances produced a mixed genotype known as a Biliary Constitution, where the person inherits the strengths and weaknesses of both the brown and blue eyed constitutions.
We are therefore left with 3 distinct groupings:
the blue eyed constitution known as the Lymphatic Type;
the pure brown eyed constitution known as the Haematogenic Type;
and the combination of the two being the Mixed or Biliary Type.