an impact vs. much impact

(Grammar Questions I Couldn't Answer)

During peer-editing of an IELTS task 2 essay, a couple of the students got into a debate, and I was called on to arbitrate.

The sentence in question was: "I admit that color has impact on people’s emotions."
The proposed edit was "has an impact" instead of "has impact".

"Yes, that's right," I affirmed.

"So impact is countable then?" the other student asked.

"Yeah, sure," I answered.  "You could impact something in many different ways, so you could have many different impacts."

But then the student drew my attention to a sentence earlier in the text.
"But I don’t think color has much impact on public buildings".

"much" is the quantifier used with uncountable nouns in English (e.g. "There isn't much water left" versus "There aren't many books")

"So should I change it to 'many impacts' ?" the student asked me.

My native speaker intuition was telling me that "much impact" here was correct.  On the other hand, "an impact" definitely seemed to be correct in the earlier sentence.  So I didn't know what to answer.