Why are shadows dark irrespective of the colour of the object
Answers
They are not black. At least not completely black. Shadows will throw the opposite color of the light. If you are outside, in very bright daylight, the light will be close to a white light in color. Then the shadow will appear to be black. If it is winter and there is snow on the ground and the sun is low in the sky, the shadows will appear to be a dark blue. That's because a low sun has an orange tone and the opposite color of orange is blue. It also depends on what color the ground is where the shadow is thrown. That might affect the color of the shadow.
If you put an item (eg an apple) in a dark room, put it on a table and turn on a red light, the shadow thrown by the apple will be green. A yellow light would produce a violet shadow, a green light would produce a red shadow.
Try this sometime. This is something I used to do with my art students. It has more to do with the cones in your eyes, than with light, but it's fun.
Draw an item (I used to use a simple drawing of a bird), then color it in completely. I colored it in with violet. Hold it against a white wall. Have someone (or you) stare at it for 2 or 3 minutes without looking away. While still staring at the same spot, quickly remove the drawing. What will appear on the white wall is an image of the bird, in the opposite color. If you used violet like I did, it will be a yellow image. If you used red, it will be a green image, blue, an orange image. What happens is, by staring long enough a something with color, it burns that color onto the cones of your eyes and when you look at something white, the opposite color will appear.
This is physics, not theory. I was an art teacher and I used this with my middle school students to impress on them that art could be science, too. I loved the way they would all say, “ohhhh, awesome!”