The female Common Kingfisher is almost identical in appearance to the male.
The difference between the two lies in the colour of the lower mandible. 
In the male, it is black, or in some cases, mostly black. 
In the female, it is orange-red, or in some cases, mostly orange-red.
Young Common Kingfishers have brown legs while adult Common Kingfishers have red-orange legs.

1st, 2nd & 3rd photo: A female. Lagoa de São Lourenço, Algarve, Portugal, 19 November 2017
4th photo: A male. Choupal National Forest, Coimbra, Portugal, 9 June 2023