Making subtle color shifts with curves

When looking at a scene, we may pick up subtle cues from the way colors shift between different image regions. For example, outdoors on a clear day, shadows have a slightly blue tint due to the ambient light reflected from the blue sky, while highlights have a slightly yellow tint because they are in direct sunlight. When we see bluish shadows and yellowish highlights in a photograph, we may get a "warm and sunny" feeling. This effect may be natural, or it may be exaggerated by a filter.

Curve filters are useful for this type of manipulation. A curve filter is parameterized by sets of control points. For example, there might be one set of control points for each color channel. Each control point is a pair of numbers representing the input and output values for the given channel. For example, the pair (128, 180) means that a value of 128 in the given color channel is brightened to become a value of 180. Values between the control points are interpolated...