Mr. Wilson’s hair color is important to the plot of the story, and so is his pride.
Mr. Wilson is “a very stout, florid-faced elderly gentleman with fiery red hair” and “small fat-encircled eyes.” He is not described as very bright. He is also evidently a proud man, from his description.
The portly client puffed out his chest with an appearance of some little pride and pulled a dirty and wrinkled newspaper from the inside pocket of his greatcoat.
This is important because only a proud and slightly conceited man could have fallen for the ridiculous scheme of the Red-headed League. Mr. Wilson is just full enough of himself and just dumb enough to be easily convinced that he can be hired just because of the color of his hair for a job that involves nothing more than copying an encyclopedia for a few hours a day.
If Mr. Wilson had not been so gullible, John Clay would not have been able to trick him. The fact that he is fat and has bright red hair makes him not very good-looking and also an easy target.
Sir Arthur Conan Doyle created the character of Jabez Wilson to suit the needs of his plot. Watson describes his first impression of Wilson as
a very stout, florid-faced elderly gentleman with fiery red hair.
The fact that Wilson has fiery red hair is, of course, essential. It gave John Clay and his partner, who also has fiery red hair, the idea of creating a "Red-Headed League" in order to get Wilson out of the way so that they could both work on their tunnel. But the adjectives "stout," "florid-faced," and "elderly" are important for one special reason. Clay gets employment as Wilson's assistant and, according to Wilson,
“Never was such a fellow for photography. Snapping away with a camera when he ought to be improving his mind, and then diving down into the cellar like a rabbit into its hole to develop his pictures."
Clay, who calls himself Vincent Spaulding, is digging a tunnel right under his employer's nose. What is to prevent Wilson from going down into his own cellar out of curiosity, just to see what is going on? What prevents him is that he is in no physical condition to be climbing up and down steep and dangerous cellar-steps. The fact that he is florid-faced suggests that he has high-blood pressure because of being overweight and would be in danger of having a stroke when he climbed back up the steps. We all know what these old cellar steps are like. They are steep and made out of cheap, unpainted wood. They are usually very poorly lighted, and they may not even have a bannister.
It's a good thing for Wilson that he never ventured down into the cellar. John Clay is a very dangerous man known to have committed murders. If Wilson found Clay digging a tunnel and saw mounds of dirt in his cellar, Clay would have no compunctions about killing Wilson on the spot, probably using the shovel with which he was digging. Then Clay would probably close down the shop and tell people his employer was away on a business trip. Clay and his partner could work undisturbed for ten or twelve hours a day and get the tunnel finished quickly. It apparently takes them about eight weeks to finish it when they can only work at it four hours a day while Wilson is at the League, but with Wilson permanently out of the way they could complete the work in less than half that time.
There is a fourteen-year-old girl who does the cleaning and some light cooking. She lives on the premises. Clay might simply discharge her. It would be easy enough to dispose of Wilson's body, since the two crooks have all the tools they need for digging.
Watson also notes that Wilson owns good clothing but that it is worn and not very clean. This helps to characterize Wilson as a man who doesn't go out much and doesn't like to spend money. It is Wilson's love of money that motivates him to come to the great detective in the desperate hope of somehow salvaging his job--or at least finding out why he lost the very nice income of four pounds a week. It is mainly Sherlock Holmes who notices some of Wilson's other features such as the tattoo and the Chinese coin indicating that Wilson has spent time in China. This seems intended to explain why Wilson might never have heard about the Red-Headed League when it was supposedly inaugurated. Clay tells Wilson that it was big news at the time and implies that most people in London know about the League.
Jabez Wilson was created to fit the needs of the plot. He is not very bright. He likes money, and he needs money. He is stingy, which explains why he hires John Clay, who offers to come at half-wages. It also explains why Wilson comes to Sherlock Holmes with his troubles. He has heard that Holmes is known to take cases without charging a fee--that is, pro bono. Wilson describes himself as a very "stay-at-home" person, which suggests that he might never have had occasion to ask other people what they knew about the Red-Headed League.