The larger it can be, the better, because as the size goes up, the amount of light collected goes up. The primary mirror reflects the incoming light to a focal point. In refracting telescopes, large lenses bend or refract the light to a focus, but these telescopes are much more difficult and expensive to build and can only be about 1 meter in diameter. By contrast, the largest reflecting telescopes are now 10 meters in diameter. The spectrum Light is the only source of colour in our world.
White light is a mixture of all colours.
Each colour has its own wavelength and energy. When white light passes through the glass of a prism, the prism bends the different wavelengths of light, each to a different degree. Violet bends the most, and red the least, with the other colours falling in between. The light emerges from the prism broken into the spectrum, all the colours of the rainbow side by side. We only see a very small portion of the electromagnetic spectrum.