I have a button image in a PSD file. The image is in single layer.
I want to change the color of the border to a bright green color.
How do I do that in Photoshop?
Here is a picture of the problem I have adressed (repeatedly) in the comments, which I believe to be the biggest issue for recoloring:
I still think recreating the rounded rectangle plus adding border is the best way to go. The original gradient can still be used as a texture for the button. Only the drop shadow has to be recreated as well.
I'm not sure why people are telling you it can't be recolored because it can be. The quality of the image and its alignment has nothing to do with whether or not it can be recolored.
To answer how the dark color shade happened in the lower portion of this button it by using Bevel/Emboss options on probably a duplicated layer so that the original creator had more control over the opacity. That is found by creating say a rounded rectangle as in this example and going to layer > style> bevel/emboss.