壁は内側が白くて外側は緑です, why 内側が白くて use が and 外側は緑 use は

Question

why 内側が白くて use が and 外側は緑 use は

why not 壁は内側は白くて外側は緑です or 壁は内側が白くて外側が緑です?

Answer 1

That's because the sentence wants to emphasize that it is 内側 that has 白 color.

If you just want to tell information about wall colors, the following sentence is the most natural.

壁は内側は白くて外側は緑です

English equivalent is "The inside wall is white, the outside wall is green."

If you are asked which side of the wall is white, then you'd like to emphasize it is 内側 that is white, so the following sentence would be natural.

壁は内側が白くて外側は緑です

Engish equivalent is "It is the inside wall that is white, and the outside wall is green."

If you are asked which side of the wall is green and which side is white, then you'd like to emphasize both 内側 and 外側. So this would be natural sentence.

壁は内側が白くて外側が緑です

English equivalent is "It is the inside wall that is white, and it is the outside wall that is green."

Answer 2

壁は内側（　）白くて外側（　）緑です。

If I’m asked to fill in these blanks when I’m given no context, I would probably put が in both, to try to make the sentence as neutral-sounding as possible.

Putting は in both would not be incorrect but the sentence sounds as if one side of the wall is supposed to be put in contrast with the other, but it isn’t.

は fits better in the following sentence, which clearly puts the two sides in contrast with each other.

壁は内側（　）白いですが、外側（　）緑です。

Now, in the original sentence, one is が and the other is は.

壁は内側が白くて外側は緑です。

Although the nuance is very subtle, it sounds as if some part of the wall being white is assumed or somehow expected, whether because of prior knowledge or common sense, and the sentence is stating it’s the inner side that is white. The outer side being green is given as additional information, somehow put in contrast with that.

By the same token, the version with two が’s could be interpreted as a statement given when it is already assumed that some part of the wall is white and the part that is not is green.

But I would say the nuance is very subtle.