i have a fairly decent working knowledge of jquery / javascript, but i can't seem to figure this one out. Can anyone that has a deeper knowledge provide me a description on how they are handling the background color transitions when a new section enters the viewport?

https://fbf8.com/

I'm currently working on a site and I intended to have this effect, and in researching i found this site as a perfect example. Only trouble is I'm struggling to figure this one out. Thanks in advance for any help!