Today I went over the background with a variety of tones, most of them green that I had mixed up. Using a colour from the tube would have given me a flatness I wanted to avoid.

I did not fully render the cup, which was a mistake. The reflection of the cloth on the cup is important, and I should have started to set that up early. What I did was mould the cloth with several tones.

You cannot darken a colour with black, unless you know what you are doing and why. Black makes things dull. Consider a line of changing tone that is all in shades of grey. It goes from light to dark. If you take a colour that is off the scale, and add black to it, you are bringing it towards that line, which means you are bringing it away from what it was. Instead, you can mix up a colour that seems to the eye to be the same but darker. This is what I was doing, trying to cover the black with a very dark green. I softened the background and kept the foreground precise, with smaller, less swishy strokes.