Besides the standard black-and-white film, he also took a couple of color rolls with him, which were not widely in use at the time.
Despite taking hundreds of pictures that day, only the two rolls of color film remain. A few days after arriving in Prague, Hajsky says, Soviet troops raided his news agency, seized all the photos of the invasion that they could find, and "burned them to embers."
The only reason the color rolls survived was because they took so long to develop. When the surviving prints came back from the laboratory a couple of weeks later, they were quietly shelved. Hajsky didn't actually get to see the pictures he took until more than 21 years later, after the Velvet Revolution in 1989.
You can watch Libor Hajsky talk about his memories of the invasion here.