Science fiction relies heavily on allegory and symbolism to generate its themes and warnings. I, Robot the film plays upon heavily-used symbols of the genre to create a morality play.
At the beginning of the film, the "good" robots are white and wear a human mask with an agreeable expression. The "bad" ones are metallic-colored, show rust, and do not wear such a human-like mask.
Later, when these good robots begin attacking Will Smith's character, their hearts glow red, a traditional indication of sin and corruption. Their facial expressions turn aggressive, even though their vocal patterns remain monotone. The symbolism connotes that newer, human-like techonology is good, but when it when we use it to replace humans, it becomes evil.
The movie is very biblically symbolic, and there is much Christian allegorical criticism of the film on the internet.