That's awesome. And it seems to be a healthy vivid yellow which isn't all that common so that's pretty cool too. I love how bettas seemingly change color as they mature. My Gyarados used to be a very plain red with only a tiny smidge of blue. Now he's red, blue, and black.
And currently now this:
I love the yellow on your little guy. I usually see the opposite happen--added color, not turn into one almost solid color.