According to Wikipedia, a colour cast is a tint of a particular colour, usually unwanted, which affects the whole of a photographic image evenly. Digital cameras will exhibit a colour cast under certain conditions. Specifically, shooting in the rainforest will produce images with a pronounced magenta cast as the camera tries to compensate for the light filtered by the trees under the rainforest canopy. Similarly cloudy conditions can tend to introduce a cast as well. In the image on the right you can see the effect on the right and the corrected image on the left. What this means is that to achieve the best colour rendition for your finished images, removing the colour cast is required.
Fortunately this is easily fixed in Lightroom or Photoshop so long as you have taken a reference image with a white balance card first. The white balance card is a neutral grey colour which is used to align the rgb components of the image and remove casts. I use a whibal for this purpose. In the image below you can see the magenta cast on my hand. The corrected image on the right has removed the cast and you can see the subtle shift in colour in the background as well.
Its worth pointing out that there are plenty of other ways to remove or modify or even introduce a cast however using a white balance card is the easiest and most reliable. Rather than explain all about it at length, the links have some good information.
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