You might not be able to see much "color" in this stage of the painting versus the previous monochromatic underpainting stage. But now I am mixing my neutrals values with a palette of color, instead of just grays.
The main difference between painting with monochromatic values and painting is color is in the transitions between light and shadow. In monochrome, you can just mix a bit of the 'light" puddle with a bit of the "shadow" puddle to make the halftone between.
When painting in color, the "halftones" is where all the most saturated color is. So each step between the light and shadow must be analyzed and mixed to match a hue/color, in addition to the value. This is very subtle when painting a monochromatic subject in color, because all the hues are relatively desaturated. But it's what makes even a monochromatic subject look like it is "in color".
Also, even in sharp edges, like where the edges of the white seashell touch the black background of the pot, the paint will look chalky and clunky. The tiny seam where the white meets the black must be knit together with a deeply saturated, dark orange or red. Otherwise the white seashell will look like a cookie-cutter shape pasted over the background, instead of a believable object sharing the same reality as the jug.
To do this, I use a small brush to push rich, saturated mixtures into the edges of the shell, and then back-fill the seashell with white, leaving a tiny thread of color between the light edge and the black background.
Since this technique is subtle and microscopic, it's impossible to see the effect in this photo. But careful attention to the reality of the edges will make the painting look believable in person.