Cleaning down
Updated
The step that protects the next batch rather than this one.
Cross-contamination is the risk
Residue from one product in the next is at best a fragrance carry-over and at worst an incompatibility: an acidic residue in an alkaline batch, or a cationic residue in an anionic one.
And a vessel left wet is a microbial culture that seeds the next batch, which is the version that shows up as an unexplained preservative failure.
The sequence
Empty fully, including tubing and pump heads.
Rinse with water, then clean with an appropriate detergent, then rinse again, finishing with deionized water.
Dry, or store wet only if it is going straight back into use.
A vessel stored wet and lidded is the worst case: warm, dark, damp and closed.
Between products
Dedicate equipment where you can, particularly between acidic and alkaline products and between fragranced and unfragranced ones.
Where you cannot, define a changeover procedure and validate it once by running a colorless batch after a colored one and looking at it.
Sequence the day to work with it: unfragranced before fragranced, pale before dark, neutral before extreme.
Waste
Do not put concentrated product down a drain. A strong acid or alkali, or a solvent-bearing product, is not domestic wastewater, and quantities matter.
Check what your local authority requires before the first batch rather than after, because the answer can affect where you can operate.
And never combine waste from an acidic and an alkaline product in the same container, which is the same incompatibility as mixing them on purpose.