Coliseum normal battle

That subroutine will cure all negative statuses, but it excludes MagiTek and Interceptor status because those statuses are intentionally set and not considered negative.

It can be done by event. The colosseum fight is at $CB78E9, if you put a hook (B2 command) at $CB7972, you can do your restore/remove HP/MP/status there. It will work for every colosseum fight except the Shadow one, which the after fight event is from $CB78F4 to $CB796B. However there is a branch to $CB7972 if you exit the item choosing menu ($CB78DD) so you might want to copy the $CB7972 code elsewhere and branch to that for this case to prevent HP/MP restore by simply entering and exiting the item betting menu. There might be a better way to do this, idk.

Ok. So I get that CACFBD remove all statues except those 2. And B2 cmd is the "Call Subroutine", so I will put CACFBD at $CB7972, right? However, the game screen does not return, so it remain black. 

But there was another objective I want to ask today. Where is the original healing code at the colosseum fight between $CB78F4 to $CB796B? The game clearly revive my KOed character, & max heal them (just not negative status), but where is that code within the colosseum codes? Is it $CB795C?

Edit: According to the FF Fandom on the coliseum. All statues should be removed except for Float, Poison, & Imp. But they didn't.

The one at CB/795C is for the encounter with Shadow only. You can see other Shadow-related things there also.

No you shouldn't put CACFBD directly over the existing code, you will need to use the B2 jump command to jump elsewhere so you can ADD the healing subroutine, which is B2 BD CF 00... as well as any previous code. How CACFBD becomes BD CF 00 will not make any sense at first - did you take a look at the event tutorials on the wiki? There's a whole section about how to use the B2 jump command in great detail, so I'm not going to waste a bunch of time trying to explain what someone else has done a detailed tutorial of.

So it seems that the Coliseum heals up the character's HP before a battle some other way than event code, I'm not seeing anything regarding healing the character before the battle. And it's definitely not that subroutine because it only heals the one person entering the battle, not everyone. The problem with healing all characters and statuses before entering a match, is that you can back out of the match just before... so you'd essentially be giving the player access to a free healing without even doing the battle. That's why it's done another way I bet. I'm not sure why they wouldn't also heal the status effects though.

If you wanted to do the healing and status curing anyway before a battle and don't mind the player cheesing it for free healing, you'd want to do it here:
CB/78D9: 5A    Fade screen at speed $08
CB/78DB: 5C    Pause execution until fade in or fade out is complete
CB/78DC: 9A    Invoke Colosseum item selection screen

This has the 4 bytes required to do a B2 jump to somewhere else there is free space, and is the best spot to do it. At the free space spot you would put:

B2 BD CF 00  (Heals all HP/MP/Statuses except M-Tek & Dog Block)
5A    Fade screen at speed $08
5C    Pause execution until fade in or fade out is complete
9A    Invoke Colosseum item selection screen
FE    Returns to CB/78DD to continue the code

This is an easy task once you understand how to do the B2 jump, which the linked tutorial covers.