With the 0, 1, and 2 thing, that's generally how digital numbering works, with "0" being technically the "first" digit to work with. In the video, that's also how the guy programmed the 5150's patches. But you can also program the 5150 so that its MIDI patch 1=green, 2=blue, 3=red. That would make it easier for me to keep track of in my head, personally. You can program it however you want in the 5150.
The point is that the 5150 III's MIDI section has a bunch of internally numbered "patches" and you can program whatever 5150 III amp channel you want to be associated with any of its MIDI patches. I'm not sure how many patches the 5150 III has, but you shouldn't need more than 3 anyway.
As far as the channel / patch thing goes. MIDI "channels" refer to the actual physical device in the rig. Like you can setup your FX8 to MIDI Channel 1, your 5150 to MIDI Channel 2, etc. When you send a MIDI command to make your 5150 go to the Red channel, the actually MIDI signal will look like:
"MIDI Channel 2, MIDI Patch 3"
or
"for any device listening on MIDI Channel 2 (so, the 5150), please select Patch 3 now."
Also, that "Channel 2" part is arbitrary. I *think* that when the 5150 goes into it's "listening mode" with blinking LED's, then it will listen for any MIDI command sent into it's MIDI IN jack and immediately assign itself to whatever channel the MIDI data notates. So if the 5150 is in listening mode and you just send a MIDI command like... "MIDI Channel 5, MIDI Patch 3" then the 5150 will go "oh I'm supposed to only listen for patch change commands on MIDI Channel 5 in the future" etc.
On the FX8, for each scene, program the MIDI Chan and MIDI PC accordingly if the device allows you to do that. I've only programmed MIDI on the Axe-Fx so I'm not 100% sure how to do it in the FX8. The official Fractal forums might be more help if this post doesn't give you the info you need.
