I had written this out in a thread but thought this information might get more visibility in its own discussion so I rewrote it for more accuracy.

Why do black and blue Sharpie's "flash" orange?

It is called "bronzing" or "sheening."

A standard Black Sharpie does not use a "pigment" in the traditional sense. It uses organic dyes. Specifically, it relies on a chemical cocktail that mimics black, rather than using a single, stable mineral. The "black" in a Sharpie is actually a subtractive mix of colors.

If you were to perform a "Forensic Chromatography" test on Sharpie ink (by placing a drop on a coffee filter with a bit of alcohol), you would see the "invisible" truth: the black ink would separate into purple, bright blue, and sometimes even yellow or red. 

If blue Sharpie, the base blue is something like a Triphenylmethane dye, known for its intense brilliance. The "Forensic Chromatography" test described above would yield cyan or even violet at the edges, added to provide the "Navy" or "Royal" to the blue.

If brown Sharpie, it is a mix of red, yellow, blue and/or black (or "black") dyes. If purple, it is a mix of rhodamine (magenta/pink) and phthalocyanine blue.

These dyes are dissolved in a propanal carrier with a resin binder. Sharpies are loaded with an extremely high concentration of dyes. When the alcohol carrier/solvent evaporates, it leaves behind a very thick layer of dye  molecules.

Normally, light travels into the ink, hits the substrate (photo paper), and reflects back, showing you, say, "blue." But because the dye layer is so dense, some light never makes it inside. It bounces directly off the surface of the dried ink. 

The molecules in blue and black Sharpies have a physical property where they reflect complementary colors at high angles of incidence. For blue ink, the complement is orange/yellow. When the light is strong enough, you see that surface-reflection "glare" instead of the color of the dye itself. The resin dries into a plastic-like film. In direct light, this film acts like a mirror, catching the light and highlighting those orange-reflecting molecules.

If the "ink" doesn't flash but recedes into the photo in the direct glare of a light it is likely a preprint. Sometimes the "ink" color is the same as the photo which is another tell.

Of course, the "flash" explained above can be accomplished with an Autopen and Sharpie...