Plastic Palette Cups make saving a bit of paint a snap to match my colors to perfectly the next day.
You may have noticed from following me on instagram or Facebook that I work in sections. I try to finish each section in a day but sometimes this is not possible with an intricately detailed dress that appears on 3 different panels or a large sky. So in order to match my colors perfectly the next day I save a bit of paint from the current session. Saving the paint and using it to compare to while mixing new paint ensures that I have consistent color from panel to panel. (On a side note: unless you fill the container all the way the paint will continue to dry, but remains moist on the inside for months. That is ok.) Just pierce the skin with a palette knife and place some on your palette to mix your fresh paint to match. It's easier to perfectly match the hue, value and chroma of wet paint vs looking at my painting and trying to color match to dry paint. Matching the colors perfectly ensures that I don't have to go back and waste time repainting sections due to inconsistent colors on the different panels.
Why not just save the paint from my current session? First, the paint, being exposed to the air all day has already began the process of oxidation. If I use it the next day the consistency is very far from optimal if not unusable due to this oxidation and I would have to add additional oil to return it to a consistency that will yield good results. That extra oil could cause additional yellowing, and would be inconsistent in sheen to what was painted previously. Not to mention I would have to try to remember what sections had extra oil added to them and which ones didn't so that I could adhere to the fat over lean rule. Ugh. But, most importantly it will not form as strong of a bond to the previous layer or ground because some of the cross linking that gives a paint film strength has already occurred due to this oxidation. So, I use fresh paint every day.
(Blick doesn't seem to offer this smaller size anymore but they have a 1 oz version that is slightly larger and will serve the same purpose.)