If you are experimenting with taking ambient lit photos -- photos without a flash -- indoors, you may notice that the colors often come out very yellow. This has to do with color temperature and white balance, and is very easy to fix both in-camera and in post processing.
All light has a color temperature that affects how it appears in photos and on video. Outdoor sunlight is very "cool" and close in color to what is output by you camera flash, while indoor lighting is often very "warm". Our eyes are very good at adjusting to these differences in color temperature, but cameras are not so advanced and will pick up on these warm and cool color temperatures. The result is that you see a yellow color cast on indoor ambient light photos when the white balance has not been set accurately.
In order to fix this yellow color cast in-camera, you must adjust your white balance setting. You are probably set to auto white balance, which, depending on your camera model, can do a poor job of adjusting for warm color temperatures. How this is done varies from camera to camera, but there are thee basic ways of adjusting white balance:
Some live view-equipped DSLRs have a handy function that allows you to dial the color temperature up and down while instantly previewing the results on the LCD.
Point & shoot cameras that do not allow you to directly change the white balance usually still have a shooting mode optimized for these kind of light conditions. It is often named something like "Indoor photos without flash".
Setting white balance in camera is great if you are shooting JPEGs, but many RAW shooters prefer to leave the camera on auto WB and correct the colors later in post production. The easiest way to do this is to use your software's white balance presets: Lightroom, Adobe Camera RAW, Aperture and most other software have presets you can choose from, which will include a "Tungsten" white balance setting. I have also found that most software's "Auto" white balance algorithm does a better job that the in-camera auto white balance.
Many post processing programs also have an eye dropper for correcting colors. Choose the white balance eye dropper, click on a neutral gray area of the photo and the software will be able to correctly read the scene's color temperature and adjust the white balance accurately.
In the following version of this article's test photo, I used Adobe Lightroom and manually chose a temperature of 2400K and tint of +25. I arrived at these settings by dragging the WB sliders until I thought the image looked correct.
The Photoshop Levels Tool can correct your white balance. Press CMD/CTRL-L and look for the three eye droppers. Choose the black on and click on an area that should be pure black, choose the white one and click on an area that should be pure white, or choose the gray one and click on an area that should be neutral gray (do one of these, not all three).
In the following version, I chose the white levels eye dropper and click the area of the wall in the center of the photo behind the cat's head.
You can also hit the Levels' "Auto" button, or choose "Auto Levels" from the "Image" menu and let Photoshop automatically adjust your colors.
Photoshop's Photo Filters can also be used to adjust color temperature. Click Image → Adjustments → Photo Filter… and choose an appropriate filter. For the following test image, I applied Cooling Filter (82) at 35%.