Our eyes, or rather our brain, has the nasty habit of changing our perception of the colour of the ambient light so that whites look "normal".
Try this experiment: Close your eyes and face directly into a very bright light source. A good opportunity to relax and take in some sun for two minutes! Now look at a piece of paper or something white. Notice how it seems blue-ish? The blood in your eyelids acts as a red filter, so brain compensates by changing the perception of colours to be blue. Or cyan to be a little more pedantic. Red and cyan are complementary colours, meaning that are opposites. (Okay, I may need anatomical correction on how the eyes and brain actually work in this scenario, but the experiment works.) https://en.wikipedia.org/wiki/Complementary_colors
What colour is daylight? Depends, doesn't it? What time of day. Which season of the year. How much cloud cover, and so on.
At sunrise and at sunset, the light from the sun tends to take on a more red appearance because the rays need to travel through more atmosphere and red, the longest wavelength, is less scattered along the way.
Us photographers call this time around sunrise and sunset the Golden Hours. Golden because of the warm, orange light. And Golden because the light source is low in the sky and softer, giving a delightful glow to our subjects.
The time before sunrise and after sunset are known as the Blue Hours. There is still sufficient light in the sky from the sun, but because it is all reflected, it is super-soft, and blue.
As the sun rises higher, its rays are less scattered, and the light becomes more intense, allowing the full spectrum of colour to reach us.
Notice the colour temperature of the elephants. Before sunrise there is quite a cool blue-ish hue, but as soon as the sun is up, the colour warms up with a golden orange. By mid-morning, the warm glow has been replaced with the full spectrum white light. As we get later into the afternoon and evening, the warm tones return. So even without knowing the specific time of day, the colours in each image give a vital clue as to when it was taken.
Indoors, man-made lights tend to fall into the "warm" or "cool" camps. Tungsten lights - those old-style bulbs also known as incandescent lamps - emit a warm, orange light. Florescent tubes on the other hand, as found in schools and other industrial environments, tend shine with a more cool blueish/greenish light.
Nowadays we have mostly done away with tungsten bulbs and the old style of florescent tubes and use CFL and LED lights to light our homes and offices. These units are still graded as "warm" or "cool" and if you install one that doesn't match the rest of the lights in the room it will look too white or too yellow, and just not right.
So how does this affect our photography? Colour generally is very important in all art as it portrays mood and emotion in the image, as well as setting the scene. A blue sunrise is either alien, or depicting an icy cold environment. We would expect a picture of a child playing in a field of flowers to have a warm, yellow glow, illustrating the joy of Summer days.
Our digital cameras have a feature called White Balance. It works something like our eyes and brain at normalising the white point of the scene. Most cameras, most of the time do a pretty good job of selecting the correct White Balance in Auto mode.
You can change the White Balance manually though. Most cameras have preset colour temperature settings for Daylight, Shade, Cloudy, Florescent, Tungsten, and a method of setting a Custom White Balance. (We won't get into that here.)
Colour temperature is measured in Kelvins. Roughly:
Temperature Light Source
1000-2000 K Candlelight, Sunrise/Sunset
2500-3500 K Standard Tungsten Incandescent Bulb
3000-4000 K Sunrise/Sunset (clear sky)
4000-5000 K Fluorescent Lamps, including cool CFLs
5000-5500 K Electronic Flash
5000-6500 K Daylight with Clear Sky (sun overhead)
6500-8000 K Moderately Overcast Sky
9000-10000 K Shade or Heavily Overcast Sky
The lower the number, the warmer, more orange the light. The higher the number, the cooler, more blue the light.
Most of the time you can leave your camera's White Balance on Auto, but occasionally you may want to force the camera to use a different setting for creative intent. Which brings us to the Challenge.
Take three photos of the same scene with different White Balance. Use the Daylight, Cloudy and Florescent options.