The only color that can be transferred to a chip in the hot-stamp process is what is on a thin plastic ribbon over the chip. The pressure and temperature of the stamping die determines how deep the impression is. The deeper the die penetrates the plastic, the wider the lines in the image are. However, nothing can add more color (gold, blue, black, etc) to the resulting image as there's only a fixed amount of colored material on the ribbon placed between the heated die and the chip.

The reason we see occasional chips with light impressions or very deep impressions is because the operator needs to test the machine setup before beginning to stamp chips in volume. You will often see multiple images on one chip, or different images on the two sides as the operator adjusts things to get the right appearance. Such setup chips were often just tossed into the waste basket when finished; those that miss the basket become the "floor sweepings" that the janitor sweeps up at night [g].