Songbirds 3e comes in several versions, and when I've tried to explain this before, I've confused folks. So, this page is an attempt to better explain it!
The original verisions of Songbirds 3e were released together as part of a Kickstarter, where you could choose to get one or both. Afterwards, on itch.io, Songbirds Blue was free to download, whereas Songbirds Red was an extra paid download.
They're the same game, but have slightly different content, flavor text, and lore implications. The easiest comparison here would be versions of Pokemon, especially where different versions change the story slightly, such as Black and White or X and Y.
Songbirds Green was a document of alternate rules, additional ideas, and planned changes to existing systems shared with Kickstarter backers and those in Snow's discord server. This was not its own PDF, and all changes from it are included in Songbirds Complete.
Songbirds Complete combined the content of Blue, Red, and Green, along with changing a lot of the general design and color scheme of the rulebook. It turned black and white instead of blue and red, and changed a bit of the vibe of the setting.
This is where we're at now! This has lots of updates and changes to clarify rules, to adjust balance and difficulty, and really update the solo play rules, among lots of other things. (The book is blue again! Yay!)
If you're new to Songbirds, I completely recommend this version! But, the really nice thing is, you don't have to stop with that version. The legacy versions are included when you buy Songbirds 3e, so if you ever want to take a look at the history of it, you can. They're all worth looking through, to appreciate the differences between them, but if you're just going to read one version, you should make it the current version of 3e.