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.