Usage of は over に to mark seasonal changes


For context, I have the following text speaking about the leaves of a maple
tree.

これは花ではありません。  
「かえで」 の葉です。  
**秋は赤いです** 。夏は緑です。

I understand that the phrase in bold would be translated as something like "as
for autumn, they are red" (and omit the leaves), but without context this
phrase may not make sense.

If the subject was included, would the phrase look like "葉が秋は赤いです"?

Would it be correct and maintain the same meaning if に was used instead?

Would it be correct to use には instead too?

Edit: The phrases above are from the tadoku stories free books so I assume
they are correct as is. I am just wondering whether I understood everything
correctly.




> _If the subject was included, would the phrase look like "葉が秋は赤いです"?_

It depends on what you want to focus, but if I had to naturally add the
subject in this context, I'd make it topic:

> この葉は秋は赤いです。

Don't worry, は can be used as many times as needed in a sentence whenever the
context fits, for example:

> このビル **は** 南側 **は** 11月 **は** ストーブをつけるほど **は** 寒く **は** ならないです。

is completely probable when their focus in conversation at that moment is the
(un)evenness of the building's insulation by season.

> 葉が秋は赤いです。

would be valid too if you don't intend to make 葉 as the ongoing topic. Then
かえで (which has whole-part relation with 葉) would become the implicit topic, so
that the full form is:

> **かえでは** 葉が秋は赤いです。

This option would be natural if your story develops like:

> 「かえで」の葉です。葉が秋は赤いです。赤くならなければ違う木です。

> _Would it be correct and maintain the same meaning if に was used instead?  
>  Would it be correct to use には instead too?_

に conceptualizes the time as a point, but adjective means a lasting state.
秋に赤い would mean "be permanently red at the moment of autumn", which explains
why it sounds bad when you mark duration of adjective with に. Using は is a
steady choice because it's like saying "be permanently red as long as (it is)
autumn".

A verb (that implies change) will accept に, so that you can say 秋に赤くなります
"redden in autumn".




> _If the subject was included, would the phrase look like "葉が秋は赤いです"?_

It depends on what you want to focus, but if I had to naturally add the
subject in this context, I'd make it topic:

> この葉は秋は赤いです。

Don't worry, は can be used as many times as needed in a sentence whenever the
context fits, for example:

> このビル **は** 南側 **は** 11月 **は** ストーブをつけるほど **は** 寒く **は** ならないです。

is completely probable when their focus in conversation at that moment is the
(un)evenness of the building's insulation by season.

> 葉が秋は赤いです。

would be valid too if you don't intend to make 葉 as the ongoing topic. Then
かえで (which has whole-part relation with 葉) would become the implicit topic, so
that the full form is:

> **かえでは** 葉が秋は赤いです。

This option would be natural if your story develops like:

> 「かえで」の葉です。葉が秋は赤いです。赤くならなければ違う木です。

> _Would it be correct and maintain the same meaning if に was used instead?  
>  Would it be correct to use には instead too?_

に conceptualizes the time as a point, but adjective means a lasting state.
秋に赤い would mean "be permanently red at the moment of autumn", which explains
why it sounds bad when you mark duration of adjective with に. Using は is a
steady choice because it's like saying "be permanently red as long as (it is)
autumn".

A verb (that implies change) will accept に, so that you can say 秋に赤くなります
"redden in autumn".

