Throughout the course of DS9, I've noticed that the Defiant either fires yellow phasers or orange-yellow phasers. This is the yellow phasers and this is an example of the yellow-orange phasers I mentioned. Why are there two different colors and which is the "correct" one?
Federation Starships can fire phasers at different frequencies. This was best described during TNG's "Best of Both Worlds" (Part 1) when Data rapidly changes frequencies to dislodge the Enterprise from the Borg tractor beam. You can actively see the color change during each shot.
Therefore it's possible, that in-universe, they were tuned to slightly different frequencies during the various battles.