Hi, I am a hairdresser so thought I might be a good person to answer your question for you.
There are 3 types of head lice.....
Eggs (also known as nits): nits are the eggs of head lice; they are small and difficult to see, especially in blonde or white hair, as they are usually white or yellow in colour. Eggs are often mistaken for dandruff and they are laid by adult head lice, usually at the foot of the scalp. Eggs are connected to the hair shaft and usually hatch after around seven days.
Nymph: nits hatch and become nymphs, which are baby lice. Nymphs look the same as lice but they are smaller. Nymphs feed on blood to survive and develop into adult lice after seven days.
Lice: adult lice are a similar size to sesame seeds, have six legs and are usually tan or grey in colour. Female lice are typically larger than male lice and lay eggs, which hatch after around seven days. Lice can survive for up to thirty days but they need blood during this time. If they fall out of the hair they usually die within forty-eight hours.
This is most probably the reason why you have found varying sizes in your grandson's hair, but as for the colour of them, that doesn't depend on a person's hair colour at all and the regularity of a child or adult getting them is just down to the sheer luck of them not catching them!
Hope this helps you x