If the image you have opened has a color cast you can use the Color correction tool to eliminate it. An example of such an image is one taken indoors with tungsten lighting. Often these images will have a yellow cast.
To use the tool click the AutoColor button or drag the mouse over the color wheel and release when you are happy with the color shift.
The example below shows how a blue TV glow is removed:
The AutoColor button balances the colors in the image so there are equal amounts of red, green and blue. This will work well for most images.
The saturation slider is used to add and remove overall color. Move the slider up to add color and down to remove it.
You can work in different parts of the image by selecting Highlight, Midtone or Shadow. This will let you change colors in the light and dark regions of the image.