As I recall, Kodachrome had a tendency to fade to redish. The other colors in the slide would fade, leaving the red base.
From memory (not always accurate) color film is in layers, each layer sensitive to a different color (RGB).
So if the Green and blue fade, you have red left.
The degree of fading would result in different colors. If the green faded you have purple.
The other is the processing.
- If the processor screwed up, the colors would come out messed up.
- Using depleted chemicals was one way to screw up the colors.
- If the processor wanted to save money, he would push the use of the chemicals beyond manufacturers recommendation; so he would process 120 rolls before changing the chemicals, rather than the recommended 100 rolls.
- If the equipment was not properly maintained there could be problems.
- Cleaning the line means stopping the line, which means no revenue from that line. So rather than clean every 10 days, he may push it to clean every 15 or 20 days.
- Was the equipment checked and adjusted for temp, processing time, etc.
When I grew up, the Technicolor lab had a bad reputation. They were cheaper than Kodak, but it was almost a flip of the coin if your film would be screwed up or not. They apparently had NO quality control, to catch and resolve problems.
Kodak was more expensive, but reliable.