The Pro Trinket does not support the Serial Monitor. Is my only option to debug code on it to upload it to another board that does support Serial Monitor first?
Use the FTDI header. It works like the Pro Mini.
Just making sure I understand...if I use the FTDI, Serial Monitor will work?
Also...if I just hack the end off a usb cable and add the right connectors will that work? I’m very new to electronics; are USB cables colour coded to a universal (as the name implies, haha) standard?
GreyArea:
Just making sure I understand...if I use the FTDI, Serial Monitor will work?
Yes. You just need to select the port of the FTDI board/cable from the Tools > Port menu. Although it's called an FTDI header you can use any USB-serial adapter that has a compatible pinout. You can get the CH340 for a bit cheaper than the FT232 and the cheap FT232 are counterfeits so you might need to jump through some hoops to get the driver working on one of those if you're on Windows.
GreyArea:
Also...if I just hack the end off a usb cable and add the right connectors will that work? I’m very new to electronics; are USB cables colour coded to a universal (as the name implies, haha) standard?
No, you need a USB-serial adapter chip between the USB and the microcontroller. This is what creates the Virtual COM Port that the Serial Monitor can talk to.
This should work. And it uses the cheaper CP2104 USB to serial chip instead of the FTDI chip but as long as you install the specified drivers, it works the same. And you get serial console output.