There is a Tinker at the DFS at the beginning of TGH, wearing green trousers and a yellow coat. Could he be Perrin's little friend?

Unlikely. In KoD Aram attacks Perrin at a crucial moment, explaining “You’re really Shadowspawn. It was you who brought the Trollocs to the Two Rivers. He explained it all. Those eyes. I should have known the first time I saw you. You and Elyas with those Shadowspawn eyes. I have to rescue the Lady Faile from you.” [KoD 30: Outside the Gates]

Since it seems improbable that a Darkfriend would be bothered by a Trolloc incursion in the Two Rivers, or be motivated to rescue someone from Shadowspawn, we consider this case closed. Aram is not a Darkfriend. Instead, we posit that he is what Dominic calls “The Shadar Logoth side of evil”, which he defines as, “crossing the line and going too far in the name of the Light, as opposed to evil of serving the Shadow.” Jordan was always very clear that extremism of any sort leads to Very Bad Things (see: Children of the Light, Shadar Logoth, Elaida, Masema) and it seems that Aram is another example of this.