Why did the color of my painting change from brownish to greenish?

Question

I applied a thin layer of oil paint and left the painting for some time.
I've been using water-mixable oil paints all the time. I had no problems with Lukas Berlin oils, which I've been using previously. This time I mixed in Cobra oils, which I've never used before. Also, I want to mention, that for this painting I didn't use any mediums, neither did I mix colors with water. Before painting, I applied two layers of gesso to the canvas.

After a couple of days, I noticed immediately that the color had shifted from light brown to greenish (see pictures below). At first I thought that this different appearance is caused by changes in ambient light conditions, but the changes are too noticeable. What is the cause for this and what can I do to prevent this from happening further on?

Answer 1

If water thinned oils behave like many acrylics there can be a slight but noticeable hue and/or tone shift to the color when the pigment fully dries. This can be more pronounced with less expensive product lines (eg student or hobby grade).

The trick these paints use is to add chemicals to the drying oil base that allow the pigments to emulsify. This allows the pigment to be 'picked up' by the water. It also adds complication to the drying behavior of the paint which is where the color change can come into play.

The Cobra paints specifically add drying compounds to the paint as well.

"With regards to drying, Cobra paints have a drier added to all colours in the range, which is unusual for an Artists’ quality paint." -source

Which can further affect how the paints behave when drying.