When one or more steps are out of tolerance it is necessary to make a careful analysis to determine the cause.
The most common cause is a change in the activity of the process.
The most likely are alterations of machine speed or developer temperature.
The effects of temperature rise and fall of machine speed are similar for black and white stocks, giving increased density and contrast.
The opposite effects are produced by a rise in machine speed or fall in temperature.
Change in machine speed or temperature can affect the three layers of a colour film differently leading to contrast mismatches.
The contrast of each layer alters independently so that instead of a grey scale from white to black the scale could appear grey with pink highlights and green shadows or other combinations. Such a mismatch is out of tolerance.
For this reason the speed or temperature of a colour process cannot be varied from the manufacturer's specification to any great extent.
The speed and temperatures of a black and white process can be varied to alter contrast