I really think it comes mostly down to direction over color.
If it's supposed to be Executor, the first shot kind of needs to be mirrored. The way it's edited, all the filmmaking cues point to it being a normal SD. If it's mirrored, it suddenly seems to be something different, because its motion becomes consistent with the direction it's finally revealed to be going.

Bash Lucas all you want, but he typically holds well to basic editing structures, like general preservation of motion, (I'm not sure what the official editing term is, but it's a basic concept). For example, picture how the X-Wings going down the trench always are shown to be flying from right-to-left. And, more recently in RotS, almost all of the opening Coruscant battle has good guys flying left-right and bad guys flying right-left, with only a few exceptions. This sort of motion consistency is a basic rule of good editing, and - if that bridge tower is supposed to be Executor - a simple horizontal flip fixes everything.

Even though every single clue (other than color) points to it being a normal SD, I'm fully willing to accept it as Executor. I really can see how it could be the Executor. But ONLY if it's flipped in the first shot. Otherwise, It's like... 3 to 1 in favor of it being a normal SD.