Left over paint on the palette? As long as paint left over on the palette has not had any water added to it, it is possible to save this paint. I keep a tub with a good air-tight screw lid and I scrape left over paint into this at the end of a painting session. Of course the resulting colour is grey as mixed together all the different colours cancel each other out, but it is good quality paint and useful for underpainting etc.
Above is a photo of my current shade of grey being used to cover an old painting as I intend the new painting to be low-key. Sometimes the grey is pinkish, sometimes it is greenish and so on, but it is always good paint.
Never add paint that has had water in it to this tub as this will only make that paint go off. Have you ever experienced the smell of off paint? What a reek! It is the dissolved solids in tap water which makes paint and mediums go bad over time (unless you use water that has been through reverse osmosis treatment which removes these dissolved solids or use demineralised water). I have had my “grey” jar for years and it is still really good paint, even though so many shades of acrylic paint have been added to it over such a long time.
There is more information in my book “Using Chroma Art Materials – Tips and Techniques for Creating Art” available for purchase from the home page on my web site triciareust.com.au.