[SDPhantom]

From what I see in the ArenaUI code, you'll need to stop ArenaEnemyFrame_Unlock() from clearing the color lock. You also need to actually check if ArenaUI is loaded since it is LoD. Here's code I would suggest running.

local function HookArenaUI()
    hooksecurefunc("ArenaEnemyFrame_Unlock",function(self)
        local color=RAID_CLASS_COLORS[select(2,UnitClass(self.unit)) or ""];
        if color then
            self.healthbar:SetStatusBarColor(color.r,color.g,color.b);
            self.healthbar.lockColor=true;
        end
    end);
end
 
if IsAddOnLoaded("Blizzard_ArenaUI") then
    HookArenaUI();
else
    local hookframe=CreateFrame("Frame");
    hookframe:RegisterEvent("ADDON_LOADED");
    hookframe:SetScript("OnEvent",function(self,event,arg)
        if arg=="Blizzard_ArenaUI" then
            self:UnregisterEvent(event);
            self:SetScript("OnEvent",nil);
            HookArenaUI();
        end
    );
end

Optionally, if this is all the code you'll have in an addon, you should add these to your ToC and use the following snip of code from the one above.

## LoadOnDemand: 1
## LoadWith: Blizzard_ArenaUI

hooksecurefunc("ArenaEnemyFrame_Unlock",function(self)
    local color=RAID_CLASS_COLORS[select(2,UnitClass(self.unit)) or ""];
    if color then
        self.healthbar:SetStatusBarColor(color.r,color.g,color.b);
        self.healthbar.lockColor=true;
    end
end);