We read in today’s Mishnah “From when may we recite the Sh’ma in the morning?. From when there is enough daylight that one can distinguish between blue wool and white wool.”
And in our Gemarah the Rabbis ask: “What is meant by this?”. Indeed. And so they elucidate: “From when one can distinguish between a wolf and a dog … [and] between a domesticated donkey and a wild donkey.” “But others say from when one can see his friend who is four amos away, and recognize him at that distance.”
Perhaps I might add: when one can distinguish prune juice from coca-cola and vodka from water.