[Phanx]

UnitReaction can return nil. Here's a version based on the coloring in my oUF layout that will do class colors for players, or gray for tapped units, or reaction colors for other units, with a fallback in case of invalid class/reaction data:

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