Making parts change materials at a certain time?

I have a script that I've been using to have SpotLight's enabled at a certain time, but I'm not sure how to make it so the parts inside of a model/the parent's material changes to Neon, then back to Plastic?

b = script.Parent

local oh,om = 6,10  -- Open Time (hours,minutes)
local ch,cm = 17,30 -- Close Time (hours, minutes)

local l = game:service("Lighting")
if (om == nil) then om = 0 end
if (cm == nil) then cm = 0 end


function TimeChanged()
    local ot = (oh + (om/60)) * 60
    local ct = (ch + (cm/60)) * 60
    if (ot < ct) then
        if (l:GetMinutesAfterMidnight() >= ot) and (l:GetMinutesAfterMidnight() <= ct) then
b.Enabled = false <--
        else
b.Enabled = true <--
        end
    elseif (ot > ct) then
        if (l:GetMinutesAfterMidnight() >= ot) or (l:GetMinutesAfterMidnight() <= ct) then
b.Enabled = false <--
        else
b.Enabled = true <--
        end
    end
end

TimeChanged()
game.Lighting.Changed:connect(function(property)
            if (property == "TimeOfDay") then
                TimeChanged()
            end
        end)