[Vrul]

Based on what you are using it for you don't really need a separate UnitIsPet function, also the class name used to index CUSTOM_CLASS_COLORS and RAID_CLASS_COLORS should be in all caps:

hooksecurefunc("UnitFrame_Update", function(self)
    if not self.name then return end

    local unit, color = self.unit
    if UnitPlayerControlled(unit) then
        if UnitIsPlayer(unit) then
            local _, class = UnitClass(unit)
            color = (CUSTOM_CLASS_COLORS or RAID_CLASS_COLORS)[class]
        else
            color = HIGHLIGHT_FONT_COLOR
        end
    elseif UnitIsTapped(unit) and not UnitIsTappedByPlayer(unit) then
        color = GRAY_FONT_COLOR
    else
        color = FACTION_BAR_COLORS[UnitIsEnemy(unit, "player") and 1 or UnitReaction(unit, "player") or 5]
    end
 
    if not color then
        color = (CUSTOM_CLASS_COLORS or RAID_CLASS_COLORS)["PRIEST"]
    end
 
    self.name:SetTextColor(color.r, color.g, color.b)
end)