== Cor de burro quando foge == 

 I have edited this into the article, hoping you won't remove it as "original research". First, "burro" means 'ass', not 'donkey' (jumento, jerico or jegue, in Portuguese). Burro is the male hybrid of a donkey and a mare. The female is a mula (mule). Actually the phrase does not refer to an undetermined colour, but to a colour that CANNOT be determined (since the ass has gone away). It is a humorous definition of infalsifiability. Moreover, equine colours in Portuguese are a mess, with a myriad terms that vary in definition from region to region and from animal to animal (a "baio" horse is not the same color as an ass of the same color). This adds difficulty to know the actual colour of a burro unless the animal is present.