How Green Was My Mouse by Dick King-Smith

Mr Muffin, a mouse farmer, breeds mice of every imaginable colour. He’s also a taxidermist and fits models with electronic devices to make them guard his mice from burglars. Merry Day comes to buy a high-class mouse and challenges Mr Muffin to breeda green mouse by mixing blue and yellow fancy mice. When Mr Muffin succeeds and wins the championship with his green mouse, Adam, he wins the hand of Merry Day and the two live happily ever after – but never manage to breed another green mouse!