You can use the color picker. Select an area in one image and add the selected color (usually the mean value of the area) and add it to the live samples.
Then you can switch to the second image, select an area with a similiar color and add it to the live samples, too.
Then you have two color values to work with. You can adjust the colors in the second image to a precise value of the original.
When you look at two images your eyes can be some kind of misleading. In the example above (flower and dog) the intense yellow and red color can mislead your eye on the right image: is the white area around the dog’s nose a neutral white?
A while ago I wrote about a similar approach to match skin color to a reference palette.