I mean, you could, arts and crafts is always an option. Would require fairly precise tapework to get it perfect around the cameras and mic and I wouldn't recommend touching the reverse side with that approach.
Personally, I'd rather stick with what they fab and it supports the company, but you could definitely DIY it.
I like the colored magnetic bezel. Very fun customization option that it's pretty emblematic of Framework's plug and play repairable design language. I'd buy a dark teal one too if they made one.
Fedora works out of the box. Does all the things Windows Pro can do but for free. $100 for Bitlocker? I mean if you want, or you can set up LUKS right on initial install. Better privacy, better security, no hidden telemetry or bloatware, plenty of initial set-up guides out there, and you'll learn more about your new machine setting things up than you would with Windows.
For all the random little minutiae that doesn't work on Linux (maybe a specific work programs or something), then just spin up a Windows VM in GNOME Boxes or set up a dual boot. Personally, I have an external SSD enclosure that's entirely Windows and I just boot from that whenever I actually need to do a Windows thing.
Purple would be dope. Post pics of the final build to the sub if you go through with it!