Welcome to the forum, and congrats on contracting "the betta bug"!
Based on what I can see the paleness to his colors is most likely from stress, and I wouldn't be surprised if he needs another few days to recover from the swinging temperatures.
Another major reason he might be stressed is because he's in a tank that is starting to go through the nitrogen cycle. Here's a sticky we have on cycling: https://www.bettafish.com/30-betta-f...-tutorial.html
Ideally you would add a couple drops per gallon of Seachem Prime, a water conditioner that also detoxifies ammonia and nitrite, every day until your test kits are showing 0 ammonia 0 nitrite and a low level of nitrates. If you do that, then you can change his water on a regular weekly schedule- I always recommend 25% of the water per week for most tanks- and not have to worry about the water quality inbetween changes. Otherwise, you'd need to change out water based on what your test kits are showing you. A bottled bacteria product like Tetra SafeStart or Seachem Stability can also jump-start your cycle, making the whole process much faster.
It's also possible that he's feeling exposed with how you have his tank set up. I see that you have a good number of plants, but they look like they're mostly pushed against the back wall? While they look good there, it leaves a lot of open space in the front of his tank- bettas like having a lot of plants that they can swim through, as you've noticed, and too much open space can make them feel exposed.
And of course, he might
have lost his colors because he's actually starting to marble- which means his color will keep changing to something completely new. Unless you know the genetics of the fish there's no way to be sure if he has the marble gene or not, and even then it crops up seemingly at random sometimes. I don't think it's super likely in your case, since there's other sources of stress in his life that could cause paleness, but it's something to keep track of in case you come home to a betta of a completely different color one day. ;)