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The sky is growing increasingly dark.
The sky is growing increasingly darker (and darker.)
The sky is growing dark.
The sky is growing darker (and darker.)

I need your comment on these four comparative sentences.

Pastel
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All six are pretty creepy, but grammatically correct, if that's what you mean!
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In the first pair, the 'increasingly' is redundant; 'growing' carries the same meaning. So these sentences exhibit poor style.

Of the second pair, which I much prefer, the former suggests that the sky was light a mere moment ago (relatively), while the latter suggests that it was already dark before.
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Comments  
The sky is growing dark.
The sky grows dark.


The former sentence puts a focus on the the on-going action, which is "grow". Using present progressive adds certain brightness or vivudness to the sentence.
The latter one states a fact. It's neutral.

What do you think?

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Vividness is not the central difference, however.

The first sentence reports an occurrence as it is happening.
The second sentence reports an occurrence in a more abstract way, as a recurring fact. This sentence is not frequently heard as is. It needs more, e.g., "The sky grows dark when it is about to rain.", or "In winter the sky grows dark early."

If the second sentence is uttered while the sky is actually growing dark, it would probably be in an historical setting. It would be an archaic use of the expression. (Or, possibly, a poetic use.)

"We needs must hie us to yonder castle, Friar Ambrose, for the sky grows dark."

In everyday American English the expression is simply, "It's getting dark."