Color is a tricky thing, especially when it comes to how we view color on monitors versus viewing it on a printed page in a book store.
I’ve discussed it before, but it bears repeating. There are very basic differences between not just CMYK (printed work) and RGB (monitor-viewed work) but also in how your eye perceives these differences. A book is a physical object that you can hold in your hands. Your physical book cover does not glow. It is flat. But you can angle it, bring it close to your face and flip it upside down if you like without losing any detail or sharpness (try that with your color book reader or your computer monitor; it doesn’t work… your image quickly loses color and contrast if you don’t view it from the correct angle).
I’m not a scientist. But I have done a lot of perusal of book covers in stores. And I have noticed something really important. It’s easier for the eye to see color tonal contrast in physical print than it is on a monitor or color reader. What does this mean? Well I’m going to try to explain it. Let’s take the header above this post and the full color cover. In a physical format, the red on this cover is quite easily picked out of the dark background and shows up really well. But on the monitor, it sometimes blends into the background. Why? Because the red is tonally similar to its background. In RGB, that means about the same number of pixels are turned on/off in the red as from the background. I think if you look at the cover in black and white you will understand what I mean. Look how similar the red is to the background when color is removed and contrast is the only difference.
When designing an ebook cover, you want to pay attention, very close attention, to contrast. The white text stands out in the large cover, the small cover, and the black and white cover. Not that you must always make your title white, or black. Far from it. But think in terms of light vs dark instead of color vs color.