how do i make a spot light change differnt colors of light slowly?

First, insert a script into the light.

L=script.Parent --Assuming the light is the scripts parent
W1=true
W2=false
while W1==true do
	wait(.01)
	L.Color=Color3.new(L.Color.r-1,L.Color.g-1,L.Color.b-1) --Just experiment with these numbers.
	if L.Color.r<=100 then--Change this to whatever
		W1=false
		W2=true
	end
end

while W2==true do
	wait(.01)
	L.Color=Color3.new(L.Color.r+1,L.Color.g+1,L.Color.b+1) --Make these opposite of #'s before.
	if L.Color.r>=200 then--Change this to whatever
		W1=true
		W2=false
	end
end

This is just a simple script, it changes the same every time, you can change that if you want.