What was Newton's original hair colour?

Isaac Newton with a brunette wig.

In all the paintings of Isaac Newton - and believe me, there are plenty to find, none of them are of him from before his hair turned grey. This had always made me wonder what he original hair colour would have been, whether it was dark brunette, light brunette, or blonde. The earliest painting of him was made when he was 46, several years after all his hair had started going grey. There was a picture that circulated claiming to be of Newton when he was a student at Cambridge, but it was dismissed by academics everywhere as being a false identification.

Of course, in later life, Newton wore wigs, all of which were dark brunette, which could mean he wanted to have a wig matching the hair colour of his younger days. But there are other variations, particularly when it comes to books written for children, where they depict Newton with blonde hair. He is even shown with blue eyes in some illustrations, despite all of his paintings showing his eyes to be brown.

The supposed image of Newton as a young man.

So what colour was his hair? In my opinion, I believe it was either a medium or dark brown. In an image that is claimed to be of Newton's mother, Hannah Ayscough, it shows her with what appears to be dark brunette hair. There is no image of Isaac Newton's father, so we cannot make any assumptions. However, it is unlikely Newton was blonde with blue eyes as some illustrations suggest, and was more likely brown eyed and brunette. Either way, the images of Newton without his wig shows that he had grey hair as early as 46. In Richard Westfall's book 'Never At Rest', he quotes that '...in the 1670s [John] Wickens told him it was the effect of his concentration'. In other words, Newton went grey because of concentrating on his experiments too much. Humphrey Newton (no relation) joked that it was '[the] Experiments he made so often with Quick Silver', which is supposedly where academics get the idea that Newton went grey because he inhaled the fumes of burning mercury.

The apparent image of Hannah Ayscough,
Newton's mother with dark hair.

It could be that Newton went grey early in his life because of one or both of these, or it could simply be a notion of stress. Events in his Cambridge years, particularly with the destruction of his Opticks papers could have brought on a great deal of unwanted stress, resulting in his hair turning grey. It is also quite possible that Newton's stress was brought on by grief from his mother's passing. Though in his childhood he and his mother didn't get along as well as his siblings did, he came to show kindness to her on he deathbed, which perhaps brought on an element of guilt, which could then had led to stress.

It might simply be that his hair turned grey, regardless of the circumstances he went through. Early greyness can be hereditary, and without any notes about his father's hair (being that Newton Sr died aged 36) it was entirely probable. Regardless of the reason, Newton's grey hair is iconic to his image, and it is instantly recognisable with the 17th century genius.