Why does the sky appear orangish in colour when it is cloudy at night

it is because when light( white light) falls on the clouds and after the light falls on the water droplets the light is dispersed into 7 colours (you must be knowing dis) and so the sky appears orange due to the vast difference between the wavelength of the bluish colours and orange colour

it appears red, actually. Because, Red light scatters the least due to dust particles. In the evening, distance between Sun and earth increases, and, hence, red being least scattered, sky apperas red (not ornage).