We see things using light, light is all our eyes can really see. Visible light appears to be colourless or white and although we can see this light, white is not considered to be part of the visible spectrum because white light is not the light of a single color, or frequency. It is made up of many color frequencies which is why when sunlight passes through a glass of water and lands on a wall you see a rainbow. If you shine the colours red, green and blue in a light and them overlap, you will see magenta. Mixing light where red and green light overlap, you will see yellow. Where green and blue light overlap, you will see cyan. You will notice that white light can be made by various combinations, such as yellow with blue, magenta with green, cyan with red, and by mixing all of the colors together.
Most colour we see is reflected light, like paint or dye molecules that absorb specific frequencies of light and bounce back, or reflect, other frequencies to your eye. The reflected frequency is the color of the object. There are some rules for light, for instance light travels in a straight line, the farther you are from a light source, the dimmer the light and the angle that a light hits a surface (the angle of incidence) is the same as the angle the light bounces off the surface (the angle of refraction).
The larger and closer the light source the softer the light and the further away the light the harder the light, it will have stark shadows. The character and quality of a photograph can be altered by the character and quality of light, so you need to think about how a scene should be lit, what lighting angles get good results, and what exposure settings will bring out the best detail and shading. A hard light (the light source is further away) will generate dark shadows and the direction of the light can place shadows in unattractive positions of a subject. One solution is to diffuse the light. Diffused light is softer and does not cast strong shadows. An overcast day is perfect for a lot of photography for this reason, the sunlight is diffused by clouds. the down side being that a white sky makes a very ordinary photographic backdrop.