One of the challenges of colour mixing is getting the right hue in the right quantity. I used to get stressed that I sometimes had a lot of paint left over at the end of a painting session.
If I mixed too little paint, I often did not have sufficient paint to make any changes and corrections, and had to mix extra paint anyway. Keeping the paint for another session did not work very well, as oils dry a lot quicker than I had envisaged, especially in the heat. Each painting exercise also requires different colours. (I have read that some painters successfully keep oil paint fresh in the freezer, but I don't have the freezer space.)
So I have been looking for ways of making use of leftover paint. Here are the uses I have found so far:
Exploring alternative versions of a painting: The other day, I was wondering whether a sculpture would have looked better without detailed facial features and did a simplified version in my sketchbook. (I love the texture of the un-primed khadi paper. It will be interesting to see whether and how quickly any oil seeps through...)
Treating myself to a bit of playtime: Producing realistic oil paintings takes a lot of concentration and mental energy. After a painting session, I find that I need some time to switch off before I can tackle my admin or other tasks on my to do list. Playing with materials without trying to achieve a particular outcome helps to "reset" my brain. As an added bonus, it may introduce me to colour combinations I may not have thought of.
Creating business cards and greeting cards: I love to add a personal touch, and I want some material to give to exhibition visitors. (I will be exhibiting in Portsmouth Cathedral and in the Petersfield Festival Hall in August.)