Let's talk a little about PSD layers. A PSD file is a Photoshop file extension that stands for PhotoShop Document. Basically, it's a working file. When it is saved it maintains the layers so it can be edited in the future if needed. Many people make PSD color overlay files (you can find a lot of them on DeviantArt) to help you finalize the look of your cover. In order to use them you'll need Photoshop or a program that will allow you to open a PSD file. A color overlay file can either enhance the look of your cover or it can destroy it. 

This was the cover of my graphic shop a while back

Here's the cover without the PSD coloring on it. You can see there isn't a huge difference in the look of the cover. The most noticeable difference is that the colors on the cover are less vibrant. The cover with the PSD layers on it pops more and catches your eye better. You don't need to make huge changes to make a cover more attractive.

Let's play with different PSD colorings and see what happens.

You can see this coloring is shouting romance at us. It's very soft colors with cute little sparkles. Definitely, not the look I was going for.

I have no words for this one. Yes, you could adjust the layers, turn off some of them or change the opacity to make it look better. But I've seen designers put up covers that look like this. It's busy and confusing. 

Pay attention to the look of your cover, watch for dark shadows or bright spots created by the adjustment layers. PSDs can sometimes create these issues. They can even end up hiding an element that is important to your design. So study everything with a critical eye.