Smock has already identified parts of it but I think the overall answer is
My head travels constantly at the velocity of the product of setting ablaze.
The speed of light in a vaccum is constant and denoted by c (already found by Smock)
My body is the head of a color, when the wavelength is DXC to 26C nm, they blaze.
DXC is 590 in Roman numerals, 26C in hex is 620 in decimal (found by Smock). 590-620nm is roughly the range on the electromagnetic spectrum corresponding to orange, whose head is o (also mentioned by Smock)
My suffix, are the occurrence of hitting a surface, when they are rays.
Incidence - in physics, is the intersection of a line, or something moving in a straight line, such as a beam of light, with a surface.
What am I? When you get it, I hope you didn't just utter a random phrase.
Is it a coincidence that we obtain the answer?
Coincidental is synonymous with accidental