Have you noticed a dark spot at the tips of your shirt collars?  Are certain shirts, like your blue ones, more noticeably tipped, while others aren’t at all?  Did they seem to just appear, doing a “vanishing ink in reverse” trick? What you have noticed is the adhesive from the shirt’s construction finally showing through the fabric. With wear and age, the glue used to adhere the collars for a nice rigid stiff appearance can show through, and the most common point is at the tips of the collars, where the two points come together.