As these are very large, they seem to me likely to be sinuses; the central one in red right along the midline is the superior sagittal sinus.
I think your blue traces may mark differently named sinuses, though they are all connected.
As a disclaimer, I'm more familiar with venous sinuses in the rodent brain, as they are important to avoid during rodent brain surgeries. Presumably the human ones are good to avoid when you poke things in there, too, but I haven't done as much of that, and anything I've done with human brain anatomy has already had these sorts of artifacts removed. If you need a specific anatomical reference from a cranial anatomy expert, I wouldn't rely on this answer too closely, but hopefully it leads you to where you need.