I'm confused, how is this an ethically loaded question? Persuasion is an essential element of successful product design. Companies need to sell, users need guidance in their browsing and decision making. There's a balance between business goals and user goals that needs to be met. I'd say A/B testing conversion is simply one way to gauge whether your tweaks can make the experience even more effective. This by itself does not mean you go straight into the realm of manipulation. That's what happens if you intentionally deceive your user, make them do things they did not expect or intend.
Here are some true dark UI patterns. You'll see that the intent is to deceive by hiding costs or terms until the last minute, going against common patterns on purpose, or tricking users into letting go of more information than they thought they would be. A button color is none of those things, that's more about visibility.
At my company we often do A/B testing to optimalize designs and conversion rates. Usually they are copywriting and layout alterations, but testing colors would fall under this umbrella as well. Though more specifically we would be testing the contrast of the color scheme (not the color itself). It doesn't matter what color you pick for your call to actions, what matters is how they stand out and work together with the other elements present in the design.