With a bold jacket like this, your best bet is to choose a plain cotton/linen or fine-patterned pocket square that tones nicely with your ensemble.  image: courtesy Instagram @articlesofstyle
The image shows a close-up of a man's torso wearing a gray plaid suit jacket, a blue tie, and a white shirt, with a folded white pocket square visible in the jacket's pocket.
The image shows a close-up of a man's torso wearing a gray plaid suit jacket, a blue tie, and a white shirt, with a folded white pocket square visible in the jacket's pocket.