The two partners are
Orange and Black
She is sunny, while his light does lack.
The sun can appear orange in the correct conditions. Black is effectively a lack of light.
She has a flavour, while he holds something back.
Orange is a flavour, holds something back could refer to milk in tea/coffee (Original solution) or to the ACDC song "Back in Black" (@Deusovi). (Or should that be the ACDC song "Back in Orange"?)
Although he is far more solemn, her vividness may make you cringe.
Black is worn e.g. at a funeral and in mourning. Orange is a bright vivid colour. This is used as a joke in the film "The Full Monty":
Still he goes fittingly with many others; she with her partners oft look fringe.
In fashion, you can wear almost anything with black. But red and orange, or pink and orange seem to clash. Alternatively, black has many rhymes, but orange doesn't
Oh, these two paired, how surprising, they form quite the pack!
Despite the differences they make an attractive and distinctive pairing
Heed well when you see them lest you be vulnerable to attack.
Maybe they are warning colours together, like for a wasp/hornet, although I'd consider black/yellow more typical.
Majestic creatures in some forms, conjuring gored gourds when given more range,
Tigers (or Monarch Butterflies) are majestic creatures, gored gourds refers to carved pumpkins...
They celebrate together in the season for harvest, for remembrance, and for change.
Autumn is the season for harvest, for remembrance of the 1918 Armistice, and for leaves etc dropping. It is also the season for Halloween, where orange and black are the dominant colours: http://www.thingsthatgoboo.com/halloween/halloweensymbolorangeblack.htm
Note also that the rhyming scheme of the poem is AABBAABB where...
A rhymes with Black, and B does it's best to rhyme with orange.