Mexican Achiote (annatto) Chicken - alternative to achiote
Achiote, commonly known as annatto (https://en.wikipedia.org/wiki/Annatto), is  seed from the achiote fruit. 

The seed is used for coloring more than for flavouring, often to mimic the colour of saffron. The flavour is very subtle - slightly peppery and lemony. 

Saffron would, therefore, be a suitable subsitute for the colour. It would introduce a new flavour to the dish, but generally not one that would be out of place. Alternatively some yellow/orange food coloring or leaving the achiote out altogether would be easy replacements.

Subsitutions for the flavour could be pepper, pink peppercorns or nutmeg, in very small amounts

The spice paste known as 'Achiote paste' is different, but includes achiote seed as a main ingredient. Here (http://www.bonappetit.com/recipe/achiote-paste) is a recipe for it. The annatto in the recipe can be replaced as described above