If I were u I wouldn't interpret the question as literal light v dark: i'd use it as an excuse to talk about the contrast that Keats buids up. Some ideas I came up with:
- Contrast between joy and pain throughout collection.
Ode to Melancholy: says you cannot separate the two (link to context and negative capability). The two need to be balanced out: when a "melancholy fit" occurs, "glut thy sorrow on a morning rose": i.e. balance it out with pleasure. Says that those who experience joy will eventually be susceptible to melancholy: those who can "burst Joy's grape" against their "fine palate" will eventually become one of melancholy's "cloudy trophies". So here light, in this case joy, is juxtaposed against the dark imagery of melancholy to convey typically Romantic idea that melancholy occurs with bliss in life.
- This idea extended to
Lamia: Keats's cynical view of their relationship is enforced when we learn that she claims their relationship will "unperplex bliss from its neighbour pain". We know from
Ode on Melancholy that Keats believes this cannot happen, so it enforces the idea that the two are part and parcel of life. This also links to idea of femmes fatales which Keats was concerned with (context).
- Also, contrast between movement and static within his poetry:
Hyperion (fragment) says that one must be "content to stoop" - this is "nature's law", and we are presented with the idea that change is beautiful - "fresh perfection treads". The way in which the titans are being phased out and replaced with the new order, the Olympian Gods, reflects this - the Titans become "grey-haired", "nerveless" and "listless" in comparison to the "Infant thunderers" who replace them. Contrast built up here.
- This links to
Ode on a Grecian Urn - idea that something which remains static lacks beauty. Urn is personified as "still unravished bride", and Keats ponders over whether or not the image of an unravished bride frozen on the urn would embody virginal perfection or the torture of unconsummated passion - will she be "warm" and "still to be enjoyed" or "forever panting" and "high-sorrowful"? This is another contrast that Keats builds up, and he concludes that something which remains static ends up as "cold pastoral" compared to the beauty of life, which changes constantly. "Beauty is truth..." reflects this - there is beauty in times changing. This poem also reflects Romantic poets' interest in art.
- erm.... other contrasts: dream v reality:
'Eve of St. Agnes'. Madeline pre-occupied with dream: she has "vague regardless eyes", implying the extent to which this fantasy has become an obsession. Strength of this also implied through "poppied warmth" - narcotic connotations (reflects Romantics' interest in drugs). Porphyro penetrates her dreams by "taking her hollow lute" - he has sex with her.
- Dream v reality:
'Ode to a nightingale' - wants to escape real world via nightingale's "viewless wings of poesy". "dull opiate" - narcotic again - strength of experience. Difficult to return to reality: has to be "tolled" to reality: cf St Agnes' Eve: "painful change", Hyperion: "pain of truth".
These contrasts could all be linked back to the light/dark thing, there are some more literal contrasts of light and dark, like the old man v the lovers in St Agnes' Eve: "snarling trumpets" v "tender chords of hollow lute" etc. hope that helps!