Replace Color is really a shortcut. It's the equivalent of making a Color Range selection (see "Color Range" in Chapter 8, Making Selections), then performing a Hue/Saturation tweak.
Replace Color doesn't let you do anything you couldn't do with Hue/Saturation, but if you only need to adjust a single color, it's sometimes more convenient to use Replace Color. To emphasize the point that Photoshop always offers multiple methods of achieving the same goal, Figure 6-64 shows how we'd do the correction shown in Figure 6-60 using Replace Color instead of Hue/Saturation.
Figure 6-66. Replace Color