Good and evil, left and right, peanut butter and Nutella—you can't really have one without invoking the other. The same goes for all the darkness and depression in this poem. The speaker can't really describe his depression without also going into descriptions of joy and happiness. For those parts of the poem, light becomes a useful symbol, providing a sharp contract with all grim stuff about witch's houses and devils' parties.
Lines 10-11: Our first glimpse of light is as "phantom light," which is affected by the speaker's description of the new moon. We're off to an eerie start here, but light will soon be a purely positive thing in this poem.
Line 44: Here the light is green, which we suppose is a step up from being "phantom." Green is often associated in literature with hope, but even staring at the green sky won't help our speaker out. He's truly bummed.
Lines 53-54: In these lines, we see light reaching its positive potential in the poem. The joy that issues forth form the soul is a metaphorical light that brightens up the world around us.
Lines 62-63: Once again, the speaker uses light to represent the awesome power of joy. He sure wishes he had some.
Line 71: Joy is now a "luminous [light-filled] cloud." It sounds pretty amazing to us.
Line 75: The symbolic light of joy, in fact, is so amazing that it's responsible for every color on earth. This is a figurative way of saying that, without joy, life is devoid of energy and vibrancy.