There is no right answer. That’s not what you want to hear, but that’s the truth. Why? Because the optimal button color for your page depends on context.
For example, the Hubspot test referenced above shows red outperforming green. However, the green button in that study blended in with the color scheme of the web page at the time. In other words, the button color was the same shade of green as the logo. Ergo, there was little contrast. The red button, on the other hand, offered significant contrast against the overall green design. Do you think that might have had something to do with why the red button yielded a better conversion rate?
Other tests revealed similar flaws. In one case, a green button was tested against a button that could barely be seen. Unsurprisingly, the green button in that study showed a superior conversion rate. That’s why, even with studies, there’s no one-size-fits-all answer to the question. It’s all about what works best for each individual website.
Well there you go. And it’s the same answer I give everyone: TEST!