What is the importance of the stranger being in a yellow suit?

As well as being associated with sunshine, hope, and optimism, yellow is also the color of cowardice and deceit. And it is these characteristics that are especially appropriate to the man in the yellow suit. When he first meets Winnie, the man is all sunny and friendly. But it isn't very long before he reveals his true colors. He's a devious, throughly deceitful man who will do just about anything to get his greedy hands on the Tucks' spring of eternal youth. He was only ever nice to Winnie because he thought she would help him find it.
When he finally tracks down the Tucks, he makes them what he thinks is a generous proposition, offering to cut them in on the huge bundle he expects to make from their elixir of youth. In other words, he offers them hope and optimism, which as we've seen, are associated with the color yellow. But when the Tucks refuse, he accuses them of being selfish, implying that he only wants to obtain their precious spring water for the wider good of humanity. This outburst brings out not just the man's true personality, but also another of yellow's connotations: that of deceit.