The red (often seen as purple or black here) are generally "heavies" which is a generic term for heavy minerals. Well the next question is what are heavy minerals? Glad you asked.. heavy minerals are garnets, amphiboles, magnetite etc.. they have the same mass as the Quartz, feldspar and rock fragment sand grains, but the heavies are slightly smaller in size (higher density) so they dont "stick up" as high as the Quartz et al. Because of this, they have less surface area for water to grab onto and are left behind, often called a lag deposit, through a process called selective deposition.
That being said I have seen clay that was eroded from the bluffs form a coating on the beach on both cape cod and Block I, but I'd wager what you guys are seeing are heavy minerals;
Probably more than you ever wanted to know, but then again I am the ranting and raving geologist
and MAC, sorry about the HiJack, glad to hear things are looking up!