How do I change a specific colour in a specific layer?

[AnyOldBiscuit]

I want to change the colour of drawn lines from one shade of green to another. All the green lines are in one layer, except for a few which are in another layer (I also want to change these ones). I have tried the bucket fill tool, but it ends up filling in the edges so it no longer has anti-aliasing, end, as a consequence, also fills small gaps. Somehow, the bucket fill tool worked fine to change colour when I tested it on a blank canvas. How do I change the colour without affecting the entire image? I only want to change the pixels that make up the lines.