What is the most useful practice for recreating the same mixed colour?

Correct answer: Keep a record of the proportions of colours used

The governing concept is colour consistency: the final mixture depends strongly on the proportions of its component colours. Recording the quantities or ratio makes the process repeatable. For example, if red and yellow were mixed in a 1:2 ratio, using that same ratio again should produce a similar orange mixture. A different brush or faster mixing does not control the colour balance. Keeping a small swatch with the written ratio improves accuracy further.