“She [Kate Waanders] happily mixes colours as she goes, preferring not to save mixes because she enjoys the subtle variations and new ways of seeing a colour when it is mixed again the following day.”Amber Creswell Bell, “Kate Waanders” in “Still Life: Contemporary Painters” (Thames & Hudson 2021), page 249
Having the trust in yourself that you are able to remix a colour isn’t something you’ll see on lists of top tips for colour mixing. Possibly because you can’t shortcut your way to it; it comes from knowing your paints and how they mix with one another. That comes with practice, working with a few colours and getting to know how these respond to one another until it’s instinctive knowledge.
By a few I don’t mean a dozen. I’m thinking two plus white. Then add another colour to these. Then another, and another, giving yourself time to internalise how they mix until you can do it without thinking. We don’t think it’s odd for an artist to work only in pencil, or black ink, so don’t feel that because you’re using colour you have to use the whole rainbow instantly.
Another part of trusting yourself is letting go of the need for a mix to be absolutely identical to the one you made earlier. In most cases, it doesn’t matter; on the contrary, it adds visual interest. Put the energy you’d use stressing about this into progressing your painting instead.