There are numerous ways to do any one specific job in Photoshop, but getting colors to match can be one of the more trickier tasks. In this video by Colin Smith of Photoshop Cafe, we are shown how to use the LAB color mode along with curves to get those colors spot-on.
Changing one color to perfectly match another color is a very useful skill to have in Photoshop. It's especially important to be able to do this if you want to get into commercial photography and retouching. From changing the color of clothes and shoes to hair and nails, the applications are enormous. And, despite the rather complex procedure, Smith is very clear and concise.
He starts out by showing us the colors he has chosen to match. Here, he has decided to use nature as his inspiration for a color combination in the form of a blue and yellow Surgeonfish. The image he has chosen to match it with is that of a woman dressed in a yellow top and hat, who is also holding a sunflower. So, as an example, he will change the woman's top to match the exact shade of blue as seen on the fish. The first and most crucial step in this tutorial is to change the image from RGB mode, which is the default mode in Photoshop, to LAB mode. In LAB mode, instead of Photoshop interpreting colors in red, green, and blue, it perceives color in L, for lightness from 0-100, while the red-green axis is represented by A, and the blue-yellow axis is represented by B, whose values range from +127 to -128 (basically, A and B are the colors, while L defines luminosity of the colors).
I'll leave my own interpretation at that and leave the video to do the rest, as it's best if you follow on with some images of your own.