The human eye is an excellent judge of color, but how we perceive different colors in a scene is also influenced by factors such as the intensity and quality of light present or colors of surrounding objects. Character and mood of a picture are influenced by all of these factors.
Colors in natural daylight are different at all times of the day and in any season of the year. Early morning light e.g. has a completely different color than the light later in the day, whereas in winter, daylight is much crisper than in summer.
Available light is reflected by our subject and consequently - since it absorbs the light present in a scene - this influences the way it looks or the color cast it has.
Indoors, colors are influenced by color temperature of lightbulbs or fluorescent lighting, which make subject colors appear yellowish and warm or greenish and cold.
Just like certain conventional films have different color characteristics, different cameras reproduce colors in different ways, because of the sensor's characteristics.
Since the human eye can adapt to differences in color much better than digicams it will sometimes be necessary to correct colors in an image. This can be done in your digital darkroom. Photoshop for instance has a Variations menu in which color casts can be corrected step by step to achieve a neutral image.