@nooneinparticular Said
If I've understood everything correctly, these are two separate concepts. One deals with the speed of light itself and the other deals with the relative speeds of the observer and the source.
Lets take these concepts individually. Suppose that someone is throwing a ball at 5 mph to another person. This is our proxy for the speed of light. Now suppose that at the same time both people are moving away from each other cumulatively at 2 mph. It could be that one person is moving away from the other at 2 mph, or that each person is moving away from the other at 1 mph each. This is what causes blue/red shift.
thats a reasonable analogy. but as Einstein's postulates claim, the way physics work will be identical for any observer, so if both these observers are reading the incoming ball speed at 5 mph, then the light wave cant be stretched because that would mean that the light physics was different as witnessed buy a moving observer.
The 2nd postulate claim was that there was no way to tell if you were moving or not. But if one observer saw a red shift, or blue shift then he would know he was moving, and in which direction and how fast.
Plus in your 5 mph example, each recipient of the ball would measure it coming in at 5 mph minus his own speed, or 3 mph as he is moving further away.
Einstein confirms this as he shows in many carriage and observers experiments that the light has to travel further to reach the moving guy. So the moving guy will always deduct his speed from the light speed, which is the only rational way to do it. Never the less, light will still be moving at light speed, its only the moving observer that measures it at plus or minus according to his own relative speed. This is the sane, logical way to view light, and fits in with red shift, and all the rest of classical physics.
Einstein's logic goes like this: It's 6 foot long, and it's NOT 6 foot long.