Are sheep colour-blind?

When I arrived at the airfield today the car park was full, so I had to park where the farmer often parks to feed the sheep. The sheep saw my red car parked there where the farmer often stops in a blue one and literally came running down the field bleating and asking for food!

I'm afraid they were disappointed as I had nothing to give them! But it reminded me of the words spoken by Jesus when he was challenged about his authority.
He said to those who were questioning him " My sheep listen to my voice; I know them, and they follow me." John 10v27 (NIV)

The sheep may have been mistaken about the colour of my car, we other hand do need to recognise the voice of Jesus and follow him.