Woodgrain-effect croissants are made by using a combination of a croissant dough block and two yeasted brioche doughs, one plain, one chocolate. The doughs are pinned out thin to a thickness of between 3-4 mm into a neat rectangular sheet. The dough can be sprayed with a fine mist of water, and the two doughs are stuck together and gently rolled with a rolling pin to ensure cohesiveness. The combined dough of chocolate and plain brioche are rolled up tightly together into a Swiss roll shape, wrapped tightly in parchment paper and frozen until solid.
A full recipe and more tips are available in my book, The Art of Lamination: Advanced Technical Laminated Pastry Production.