If you want to make just one cool adjustment to your photos, don’t bother with the complexity and expense of the big image editors. Color Select Studios ($15, download) is an incredibility simple program that lets you add selective color to a black-and-white image.
After downloading the program, just upload a color photo, and Color Select Studio will automatically convert it to black-and-white. Next, select the size and shape (circular or square) of the brush, and paint away where you want color to be.
There are additional two sliders, one to zoom in for more detailed selections and the other for adjusting the level of saturation. The saturation default is 100%, which is what you will most likely want, but you can use less saturation for more delicate color addition. You can paint areas back to black-and-white by using a saturation of 0%, but there are a redo and undo selection options as well.
When Color Splash saves your images, it renames it with recolor at the beginning of the file keeping the original safe. This is a fun program best for people who want to make a simple but dramatic change.