So that explains the switch to standard red. Still, I can't recall why the switch from the rainbow top to the grays in season 2... except that it obviously looked better.
In my opinion the easiest and most likely in-universe answer is that in the time frame between Seasons 1 and 2 Starfleet changed the design for actin ensign uniforms from the sweatshirt with the three division stripes to the grey number.
Just like they phased out the Skants during roughly the same period and updated the standard uniform shortly after (between seasons 2 and 3)
Of course that all just once again shows how ridiculously often Starfleet seems to change their uniforms.