This is from the 2004 press release:
Prior to being sent to Lowry’s company, the original negatives of the three STAR WARS TRILOGY films were transferred to high-definition video (in a 10-bit 4:4:4 RGB format) through the telecine process, then sent to Industrial Light & Magic, Lucasfilm’s visual effects company. At ILM, technicians worked with George Lucas to digitally “color time” the movies.