In cold weather, accelerants (not catalysts) are often added to the mix to speed the hardening process. The most common additive, though often avoided due to its reaction with reinforcing steel, is calcium chloride. It can create discolouration in the concrete, as can other accelerants and materials, even plastics and blankets used to keep the freshly poured mix warm.

Unless appearance is a factor, such as in driveways or where the surface is left exposed in modern architecture, discolouration is of no consequence.

That discoloration has to do with concrete seepage through the form work ("the cement cream"). That discoloration is pretty common where forms meet and box-outs are made (such as windows/doors.) When formwork loosens during concrete placement, it allows the cream to leak through a bit. Unless there is loose aggregate (gravel), (and I don't notice any in the pic) no worries.