How Can I Make A Script For Material Like Grass To Appear Only At Night Time?

This should work. string.sub() subdivides the string. parameter 1 = string, parameter 2 = first position, parameter 3 = second position (defaults at the length of the string).

local part = game.Workspace.Part
while true do
	wait(1)
	local l = game.Lighting
	local h = string.sub(l.TimeOfDay,1,2) 
	if h < 6 or h >= 18 and part.Material ~= "Grass" then
		part.Material = "Grass"
	elseif h >= 6 or < 18 and part.Material == "Grass" then
		part.Material = "Plastic"
	end
end