An important but generally unknown characteristic of images is white balance - what exactly is a "neutral" color (greys, whites, blacks, etc), from which all the colors can be based upon. As we discussed before, the camera's sensor gets information about how much red, green, and blue there is in the picture, and from that, the camera derives the color. Normally, we would expect equal amounts of red, green, and blue to equal white light. However, what is actually "white" and what we perceive as "white" are two very differnet things. Take a typical indoor scene, for example:
This is what we see with our own eyes. However, the fluorescent lights in the room are actually emitting light that tend very heavily towards the red end of the spectrum - there is almost no blue spectrum light coming out of the fluorescent lamps at all. In actuality, the lighting in the room really looks like this:
Wow! This is quite a big difference, and nothing like what we see in real life! Yet if you actually measured the light in the room, this is what it would look like - lots and lots of red, and nearly zero blue. The reason we see the image above is because our brains interpret the signals from our eyes and performs a sort of "white balance". We actually do perceive the "true" color image on the bottom, but upon receiving this information, our brains go: "Woah! This color is all wrong! It should look like this..." and reinterprets the lighting so we get the much more natural-looking scene above that we're accustomed to.
Cameras however, don't know anything about what colors "should be". They simply receive and interpret the light for what it actually is, which results in images like the 2nd one. As our minds re-interpret the light information to give us a more natural-looking color balance, cameras must do the same "white balancing" to produce colors that are in sync with the human interpretation of light - essentially, turning the second image into the first image.
Cameras do this via the "white balance", where either a neutral point (a white/grey/black point that should have no color saturation) is defined, or the camera is given a preset interpretation ("interpret this light as sunny/cloudy/fluorescent"). This is one of the most critical aspects of creating an image, as interpreting color the wrong way can have catastrophic results:
However, if you happened to not set your white balance correctly to begin with, there are ways to correct the white balance afterwards. The most rudimentary tool for this is Color Balance. In Photoshop you can find this under Image->Adjustment->Color Balance. In GIMP this is found under Colors->Color Balance.
This is the example original image, which as you can see has a very strong warm tint, due to the nature of the fluorescent lights in the room:
Even though the color is extremely off, we can correct it by applying a very strong color balancing - we want to offset the yellowish and reddish tones, and shift them towards more cyan and blue tones. This is the result we get:
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