White light is an equal combination of all the wavelengths in the visible light spectrum (red, orange, yellow, green, blue, indigo and violet) shown at the same intencity
The colour we see is a result of which wavelengths are reflected back to our eyes. Objects appear different colours because they absorb some colours and reflect others.
Black is the absence of light or where all of the colours in the spectrum have been absorbed leaving no wavelengths to reflect back to the human eye