[Lily.Petal]

Posted from Below and from PM's
(Big thanks to Nib!)

Normal Threat:

-- OnLoad

self:RegisterEvent("UNIT_THREAT_SITUATION_UPDATE")

-- OnEvent

-- Replace unit with "party1" to "party4" for party members. Or ("target", "targettarget") for seeing threat on your target.
local status = UnitThreatSituation(unit)
local color = status and {GetThreatStatusColor(status)} or {0,0,0}
self:SetBackdropBorderColor(unpack(color))

--------------------------------

Personal Colors: (also with extra quirks that doesn't show threat unless you're in group/have pet and won't show in BGs)

-- OnLoad

self:RegisterEvent("PLAYER_ENTERING_WORLD")
self:RegisterEvent("PARTY_MEMBERS_CHANGED")
self:RegisterEvent("UNIT_PET")
self:RegisterEvent("UNIT_THREAT_SITUATION_UPDATE")

-- On Event

--- {R, G, B, A}  - Set A to 0 to hide the panel.
local ThreatStatusColors = {
	[0] = {0, 0, 0, 0},		-- not tanking, lower threat than tank. 
	[1] = {0, 0, 0, 1},		-- not tanking, higher threat than tank. 
	[2] = {0, 0, 0, 1},		-- insecurely tanking, another unit have higher threat but not tanking. 
	[3] = {0, 0, 0, 1},		-- securely tanking, highest threat 
}

local color
local status = UnitThreatSituation(unit)
local _, InstType = IsInInstance()
if ( (status ~= nil) and (UnitExists("pet") or (GetRealNumPartyMembers() > 0) or (GetRealNumRaidMembers() > 1)) and not (InstType == "pvp" or InstType == "arena") ) then
	color = ThreatStatusColors[status]
else
	color = {0,0,0,0}	-- Hide
end
self:SetBackdropBorderColor(unpack(color))

---------------------------
Same as above, but default ThreatColors:

-- OnLoad

self:RegisterEvent("PLAYER_ENTERING_WORLD")
self:RegisterEvent("PARTY_MEMBERS_CHANGED")
self:RegisterEvent("UNIT_PET")
self:RegisterEvent("UNIT_THREAT_SITUATION_UPDATE")

-- OnEvent

local color, status
local _, InstType = IsInInstance()
if ( (UnitExists("pet") or (GetRealNumPartyMembers() > 0) or (GetRealNumRaidMembers() > 1)) and not (InstType == "pvp" or InstType == "arena") ) then
	status = UnitThreatSituation(unit)
	color = status and {GetThreatStatusColor(status)} or {0,0,0,0}	-- Threat colors
else
	color = {0,0,0,0}	-- Hide
end