First, just select a picture that you want to apply the color accent to. It should be a picture with a distinctive color in it. I have chosen the picture of Mackenzie to the right with the orange on her hat and cover-up.

Next, create two more copies of the original layer by right-clicking on the layer and selecting "Duplicate Layer..." After you have your three layers, Desaturate the top and bottom layers by selecting each one and pressing Shift+CTRL+U. You should now have three layers - the top and bottom ones are greyscale and the middle one is the original color.

Now create a layer mask on the top greyscale layer by selecting the layer and clicking on the highlighted button on the Layers palette.

Next, select the paintbrush (or the pencil or airbrush) and using black begin coloring the areas that you want to accent. You can see that I've started doing that on the picture to the right. By coloring your layer mask with black, you are essentially cutting holes in the top layer and allowing the middle layer to show through.

Then, just continue coloring areas on the mask that you want to show through until you have all of the color accented that you want. You could actually end at this point, but sometimes the color is a little intense and appears overly bright on the grayscale background.

This is the reason that we created three layers to start instead of just two. You can now select the middle layer, and dial down the opacity on the Layers palette. You can see in the example that I made the opacity 55% and allowed the bottom greyscale layer to partially show through so that the orange would lighten up a little.