Open two images; the one you want to recolor and the one you want the colors from. Next, go to the "Decorate" tab and select "Grab Color". Click on the color on an image you want to add to the other, then click "Done". Click on "Fill", click on the color you want on the other image to be changed, wait for it to change (if it's a large space of one color it'll take longer). When it's changed, click "Done" and repeat with the other colors.
Sorry if that was too much information, just wanted to be clear.
Well, some of the images I use don't like to recolor properly (like the Garnet image I used), leaving like an outline of the original colors on the edges. I don't really have a solution for it, other than either use a different image or take your time filling in the outlines as well as you can. Also if you save the image, usually the quality isn't great so the outlines are kind of hard to see, which is good. A good example is my second Malachite/Sugilite recolor.