The sunrise over Skerries on September 25th. Clearly a red sky, and therefore clearly a warning if folklore is to be believed. But it heralded a lovely day, warmer than it has been, and very settled. Which prompted a little reading....
The sky is blue due to the scattering of blue light from the sun as it passes through the atmosphere. This 'missing' blue light mean that the sun usually appears yellow to us. And at the beginning and end of the day, as the sunlight passes through additional kilometres of atmosphere, the effect is increased and the sun appears red.
But in order to see the sun, and therefore its redness, we need pretty clear skies (with maybe just a scattering of clouds to reflect the light).
So a red sky at night tells us there are clear skies to the west - where the sun is setting and where most of our weather comes from. So the red sky at night really can tell shepherds to delight. But a red sky in the morning is telling us about clear skies to the east. Where our weather doesn't (usually) come from - so it means much less.
In fact the only reason that shepherds should take warning - as far as I can see - would follow this logic: its not raining now, so it will probably rain later!
Here's a good link....
http://theconversation.com/are-red-skies-at-night-a-shepherds-delight-an-astronomers-view-85691