Astronomers call this phenomenon "scintillation." It is caused by thermal irregularities in Earth's atmosphere. Packets of relatively warm and cool air act like prisms, spreading starlight into rainbow colors. When these packets drift in front of Sirius--voilá.
The reason Recke's first exposure looks white is because 1/2 sec. is long enough to average many colors together. Multiple packets of air drift by during that time. The shorter snapshots, however, capture individual packets; they look like tiny supernova explosions.
All stars scintillate, but Sirius does so most flamboyantly because of its extreme brightness. See for yourself. Step outside this evening an hour or so after sunset and look east for Sirius near the feet of Orion. The lower it is, the more it scintillates. You may be able to see the colors--no camera required.