If you're a casual shutterbug, you might have noticed that the colors in your photos can vary wildly. Under light bulbs and natural light, everything has an orange tint. Under fluorescent lights, everything seems pale and blue. The shade of yellow you get when you shoot a flower on a sunny day might be drastically different from the shade you get if you shoot the same flower on a cloudy day.
If you don't set the white balance correctly when you take the picture, or if the colors still look slightly off, you can still fix them in Photoshop. The fastest and most simple way to correct colors is with Image > Adjustments > Auto Color, or pressing Shift + Ctrl + B. This will automatically adjust the colors in your picture to what Photoshop thinks is the best levels. Like most one-click solutions, this is only effective in certain situations, and isn't the most reliable or powerful way to fix colors.
You can also manually adjust the color levels in the Image > Adjustments > Levels menu. Under "Channel," click on Red, Blue, or Green. The graph will show a histogram of the (red/blue/green) color channel. If you see empty spots on either end of the curve, move the black or white triangle (whichever is closer to the curve) to the spot just before the curve starts. If, instead of just a short gap, there's a very long tail at the beginning or end of the curve, nudge the triangles just a bit closer to the curve and watch the colors change. If you push the levels too far, the picture can "posterize," replacing the fine gradients of color in the image with jarring bands. If manual level adjustments still aren't enough, you can adjust the color curves in the Image > Adjustments > Curves menu. The curves are even more finicky than the levels, and can do more damage than good to your picture if you don't know what you're doing.