The Executor model used for filming was grey. It was re-timed blue in post. They only built one large scale tower for ESB. So, if it was supposed to be just your regular stardestroyer then why did they re-time the colour of it to blue if it wasn't supposed to be the Executor? It makes no sense whatsoever. ?

Some more evidence. This time from a bootleg betamax 1980 telecine. Yes, the colours of this are nowhere near perfect, but you can still get a good idea of what colour the original tower was when seen in theatres.

Here we have it coming into view. You can see that the other stardestroyer is clearly a standard because it is white.

A few more frames on and yes, that is definitely blue.

Next we see the standard stardestroyer and you can tell its supposed to be white even with the red shift. Now look at that TIE. Blue, which we know they are in this film.

now cut to the reveal shot of the Executor and it's almost the same blue same blue as the tower we first saw.