Hair dye.

Whilst running away from the army and police across Europe, Tyson took the precaution of dying his hair to mask his appearance a little.

Tyson has rather sensitive skin so he took the safer route of using a natural, semi-permanent hair dye that washed out and needed refreshing after five days.

Originally he was only going to spend the time in Europe dying his hair a medium blond, which ended up being two months, but when he got back to Britain he continued doing it for another month; to be on the safe side.

Tyson has also repeatedly dyed his hair as a disguise on missions for Moriarty. To the current date, his hair has been dyed: 

• Light Brown - on three different occasions
• Light Brown, hints of Ginger - once
• Medium Blond - on seven different occasions
• Pastel Blue - on two different occasions
• 'Honey' Blond - on two different occasions
• Dark/Dirty Blond - on four different occasions
• Black - on three different occasions
• Light Ginger - on two different occasions

As soon as though missions were over Tyson washed out all of the hair dye to return his hair to it's natural colour. 

Though having his hair dyed helps Tyson looks less like his father, of whom he is the spitting image, Tyson prefers his hair to be his natural medium-dark brown [depending on how much light he'd exposed to].