Learn Hardcore Hebrew: זאת לא רק בחירה של צבע, אלא גם שינוי קטן בתחושה של החדר. - This is not only a choice of color, but also a small change in how the room feels.

זאת means this and is the feminine form. It is used here because the sentence is referring to an idea that is grammatically feminine, especially בחירה (choice), which is a feminine noun.

So the structure is roughly:

זאת = this / this is
לא רק... אלא גם... = not only... but also...

In modern Hebrew, you may also hear זו instead of זאת in many contexts.
You might sometimes hear זה used more loosely in speech, but in careful standard Hebrew, זאת matches the feminine idea better here.

How does לא רק ... אלא גם ... work?

This is a very common Hebrew pattern meaning not only ... but also ...

In this sentence:

לא רק בחירה של צבע = not just a choice of color
אלא גם שינוי קטן בתחושה של החדר = but also a small change in the feel/feeling of the room

So:

לא רק = not only / not just
אלא גם = but also

This structure is very close to English, but note that Hebrew uses אלא after a negation like לא.

A few similar examples:

זה לא רק יפה, אלא גם שימושי.
It’s not only beautiful, but also useful.
הוא לא רק מורה, אלא גם סופר.
He’s not only a teacher, but also a writer.

Why does Hebrew say בחירה של צבע instead of בחירת צבע?