Easy to confuse velocity with frequency. I think your question should be 'in what ways could c be varied for a given photon?'. Perhaps if a photon was near a black hole, we would see c alter for that case. You have some great answers which clarify here already though.

Indeed. It's such an exciting frontier :)

Anna, thanks. I guess I'm trying to envision a mechanical or quantum-mechanical process at tiny time intervals - the things that occur that we might have some control over some day that determine a quark's existence and state.

Thanks anna, your answer deserves a tick. It does however still stump me - the governing factors that determine a quark's state and existence.

Thanks @JohnRennie, its good to know someone else out there is notably stumped by this one.

I found this page: hyperphysics.phy-astr.gsu.edu/hbase/particles/quark.html which contains a table of Quarks and their respective charges/masses. Does this imply that an anti-proton may have a different combination of quarks in it, or a set of antiquarks which are of exactly opposite charge?

Thanks @dmckee for answering. Of course this makes sense. What governs the quark's state though? How does it have a charge in the first place? (sorry if this is so basic). Your answer will probably relate to the conservation of energy.

I see - so an electron pair (+-) are created and have opposite velocities on creation/exit? So energy is conserved. I'm still hung up on the proton though - it's a much larger particle. Just reading up on current production methods for anti-hydrogen.

Thanks for that, conservation of energy is a very reasonable explanation. In beta-decay there are anti-particles - this is an example of an anti-particle coming from a regular particle. What goes on in that process to allow for it?

I guess the question should have been... what is the mechanism for a particle becoming anti-matter or matter?