Learn Italian: Aggiungiamo qualche sostanza, quindi il liquido cambia colore. - We add a few substances, so the liquid changes color.

Yes! In this sentence, quindi, perciò, and dunque are completely interchangeable. They all mean "therefore," "so," or "as a result." You are stating an action (adding substances) and using quindi to introduce the consequence (the liquid changes color).

Could I use quindi to say the liquid is "so" colorful?

No, because that is a different kind of "so" in English! Quindi means "so" as in "therefore" or "as a result." If you want to say something is "so" (meaning "very") colorful, you must use così instead.

The translation says "a few substances" (plural), but sostanza is singular. Why isn't it plural?

The word qualche translates to "some" or "a few," but a strict rule in Italian is that it must always be followed by a singular noun. Even though you mean multiple substances, you must write qualche sostanza, never qualche sostanze.

Why is there no article before colore? Shouldn't it be "cambia il colore"?

In Italian, the phrase for "to change color" is simply cambiare colore, without a definite article. It is a common fixed expression, much like cambiare idea (to change one's mind) or cambiare vita (to change one's life).