What software? How can I change all of one color to another color in a JPG or a GIF?

First, I don’t have Photoshop or any expensive professional siftware tools. If I can do this, I would like it to be free/open source.

I have a background image – originally a photo – which is in various shades of blue. (Logo = dark blue, remainder of the background in shades ranging from very light, pale blue, all the way to medium blue.

What I’d like to do is keep the various shadings and variations, but change all the blues to greens, or maybe shades of red/maroon.

- what free / open source software would let me do this?

- what is this process called? color shifting?

Thanks-