Photo Editing: How to decide which color to be picked while re-coloring the images?

At one time, there were special transparent coloring paints specifically for this purpose. Today, with digital media, it's a matter of editing the image -- generally by adding various color masks or layers the blend the color with the black and white image. This could be done free-hand with a tablet or mouse painting colors from the digital palette, or by selecting areas/objects and applying a color mask. The key is typically the "blend mode" which defines how the paint program blends layers of an image. You don't want opaque colors obscuring the image, you simply want to change the hue and saturation of the color while maintaining the value. As for determining the color of things... Obviously, there's no way to know from the photograph itself, but one generally uses what ever information they have about the items in the scene to guide the process. For example, one might suspect that a clear sky is blue, or that uniforms were made of a certain color of cloth... Flowers and things have well-known colors, and some items may actually be visible today in storage or a museum. In short, one uses whatever information they have about the items in the scene to guide color choices, then makes up the rest.

In case of that car you can choose which ever color suits the scene.  In most of the cases when an image is to be colored, usually they talk to the subject, or in this case the photographer. There are also other methods to determine, like understanding the saturation of the image, looking of reflections, effect of a particular colored object on the surroundings. So considering all these aspects we can decide which color is most suited for a given object. For instance, you can make a person look like people in avatar, and it's not wrong way of interpreting. Its just a different perspective to the image. Your perspective.

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