My theory is it's a direct epilogue.
Assuming the virus is widespread or at least still a threat, Sydney and Grant survive in what could be called "the language apocalypse" by maintaining a kind of improvisational roleplay. This allows them to express ideas in a freeflow that inhibits deeper understanding and prevents them from infecting each other.
However, the efficacy of this roleplay is limited, as show in the shift from black and white to color (fantasy to reality; safety to danger). The fake conversation bleeds into a real one the longer it stays on topic ("let's get out of here" is on topic but followed by abstract, noir-ish improvisation... until it gains too much context, becomes "understandable" and dangerous?), represented by color bleeding into the scene.
That conversation itself is a threat by being coherent, but could be precipitated by a realization that they're in an infected zone where roleplay won't cut it. Or it could be playful banter about getting the cheque and going home that gets too real and ends up compromising them.
Either way, the scene congeals into understandable communication and ends with the big taboo: a term of endearment uttered twice ("baby--" "shhh!!"). The film established several times that terms of endearment are carriers for the virus. Grant slips up saying it once and by now we're in full color. When Sydney says it he shushes her. This tells me the virus is still a present threat. Hence the roleplay.