we are able to see the colour of a red dress because out of all the components of white light red gets reflected the most.in that case how can we justify the black colour being visible when it is not even a component of white light?

Black objects are black because when light rays hit them, they absorb every color from the spectrum, which makes them appear black. So even if you put them under a different colored light, they will still appear black. White is actually the exact opposite, since it reflects every color in the spectrum.