Chlorine is greenish yellow under the light of a yellow sun. Under the light of a hotter, bluer, sun, it will look different. Of course, the aliens might not be breathing chlorine gas. With no free oxygen, you could have seas of carbon tetrachloride, used in the same way as chlorine gas would be.
Humans have occupied different food chains, while spreading across the planet, and encountered many novel biotoxins, but we have not speciated for that reason alone, contrary to what you are suggesting. Carbohydrate/protein based alien life isn't going to chemically any stranger than what we've already encountered on this world.
Nor would even major genetic changes necessarily prevent inter-breeding. During our evolutionary history, the number of chromosomes has changed, yet the proto-people with more could still breed with those with less.
Quite simply, considered in the light of everything we know about biochemistry and evolution, speciation is not plausible under the circumstances described unless the precursors were deliberately trying to produce multiple species - which they may well have had reason to do.
Humans have occupied different food chains, while spreading across the planet, and encountered many novel biotoxins, but we have not speciated for that reason alone, contrary to what you are suggesting. Carbohydrate/protein based alien life isn't going to chemically any stranger than what we've already encountered on this world.
Nor would even major genetic changes necessarily prevent inter-breeding. During our evolutionary history, the number of chromosomes has changed, yet the proto-people with more could still breed with those with less.
Quite simply, considered in the light of everything we know about biochemistry and evolution, speciation is not plausible under the circumstances described unless the precursors were deliberately trying to produce multiple species - which they may well have had reason to do.