In this article from Threads #190 (April/May 2017), Sharon Wilkinson shares her method for creating less bulky collars with a trim. She writes:

This collar construction method is a nifty trick I came up with that minimizes bulk at the collar edges while adding a decorative trim to define the collar shape. The key feature of this technique is that it gets rid of the seam at the collar’s edge. It is also a relatively simple way to add contrasting bands to the collar edges. Depending on the fabrics you choose, you can create the effect of one or two bands of contrasting trim along the collar edge. It’s the perfect opportunity to play with print- and color-mixing for shirts with personality.