Nits are the empty shells so though they are a bit unsightly they are completely harmless. They appear a white/cream colour depending on the shade of hair. Head lice eggs are appear the same colour as the hair as they reflect the colour of the hair because of the fluid inside the egg, incubating the baby (nymph) louse.
But why are head lice eggs and nit so hard to remove? The fact is the eggs are not just sitting on the hair. The female louse excretes a liquid (similar to keratin - which is the same protein structure as hair - this is why you cannot dissolve an egg or nit - if you did you would damage the hair? The liquid forms a cylinder shape that is cemented to the hair and then the egg sits on the cylinder/straw. Please see photo
This means the whole egg and its straw need to be drawn or pulled all the way down the hair shaft and off at the end.
Nits are even harder to remove than the eggs because as soon as the nymph hatches and leaves the shell the walls collapse in on themselves making the nit even smaller. When heat is applied to the hair this will shrink the nit even further.
Most nits will fall of eventually but this can sometimes take anywhere from six months to a few years.