Here's a poser for you. Below are the evangelist portraits from the famous Lindisfarne Gospels, reputedly painted (according to a 10th-century colophon) by Bishop Eadfrith (698-c. 721).

The question is: can you spot the difference? The answer is found at the foot of this post.

St Matthew the Evangelist in the Lindisfarne Gospels (London, British Library, MS Cotton Nero D IV, f. 25v)

St Mark the Evangelist in the Lindisfarne Gospels (London, British Library, MS Cotton Nero D IV, f. 93v)

St Luke the Evangelist in the Lindisfarne Gospels (London, British Library, MS Cotton Nero D IV, f. 137v)

 St John the Evangelist in the Lindisfarne Gospels (London, British Library, MS Cotton Nero D IV, f. 209v)

To find out more about the making of the Lindisfarne Gospels, we highly recommend that you read Richard Gameson's new book, From Holy Island to Durham: The Contexts and Meanings of the Lindisfarne Gospels (Third Millennium, 2013). Or why not visit Durham itself, where the manuscript itself is on display until September 2013? Meanwhile, don't forget that you can view all the pages of the Lindisfarne Gospels on the British Library's Digitised Manuscripts site.

So, the answer to our poser is ... well, it's a bit of a trick question. All the eyes are blue, except ... you'll have to work that one out!