The approved physical sample that defines a production colour, kept in store and almost never handled; working standards are duplicates of it used for everyday comparison.
Instrument makers' guidance is to make identical duplicates when a colour is approved, archive one as the master, and use the others one at a time as working standards, replacing each as it fades or is damaged. Neither has a fixed life: a standard is valid for as long as it still measures as it did at approval.
A master standard is not the calibration tile supplied with an instrument. The tile checks the instrument; the master standard defines the colour a product is supposed to be.
A paint maker keeps the signed-off panel for a colour in the dark and issues a duplicate panel to the production laboratory.
If the reference itself drifts, sound batches start to fail in one direction and nothing in the process explains it.
A standard is checked by measuring it again and comparing the result with the values recorded on the day it was approved. Guidance from instrument makers is to verify periodically in place of setting an expiry date, because a certified white tile is built to stay stable for years while a standard cut from production material may not. Temperature has to be controlled as well. For one published set of twelve ceramic tiles a 10 °C rise moves each grey tile by no more than 0.04 in any CIELAB coordinate but moves the red tile by about one ΔE*ab unit, enough to use up a tight tolerance before any batch is measured.
Batches begin to fail in the same direction although nothing in the process has changed.
The working standard may have faded or been soiled, so the reference has moved and the batches have not. Measuring the standard against the master separates this from a changed raw-material lot, which produces the same pattern.
A standard is measured straight from a cold store, or a red or orange sample is read while still warm.
Some colours shift measurably with temperature and shift back when it settles. A reading taken off temperature compares the batch with a reference that is temporarily a different colour.