Before you do this exercise, look at a lesson about painting shadows on an opaque fruit by David Leffel (link). Now, to learn how to paint translucent objects (grapes), start at the upper left in the image above then move clockwise. This is adapted from "Oil Painting Secrets From a Master" by Linda Cateura who shares teaching from David Leffel.
1. Put a thin middle grey background ( I used transparent oxide red and ultramarine blue and wiped it back to get thin color.) For the "tabletop", put a small amount of a lighter neutral color. (This is lighter because a tabletop is a horizontal surface and "collects" more light.) Now paint a single value purple or red form. Notice that the flat value and the hard edges make this red/purple form look like a polka dot floating on the grey background. Our light source for this exercise is to the left as you look at the images.
2.Now, give some weight to the polka dot by adding a shadow
3. Add a highlight and the magic. On the side of the object that is farthest from the light add WARM (orange, orange-yellow or orange-red) where the shadow would be with an opaque object (see the last figure lower right to compare the form shadow of an opaque object ). Translucent objects have the light pass through them and then reflect back creating this warm area furthest from the light source. This is different than an opaque object that has a shadow where the light does not reach the far side of the object (Remember that this form shadow is a concave shape)
4. Modify the edges: soften the cast shadow as it moves away from the object, soften the transition between the warm area and the red/purple. If your exercise is large enough, you can change the edges of your object (making them slightly cooler) which will make your object appear rounder.
On the right area are examples of exaggerated warm colors in the transparent objects. A master of exaggerated translucent glow is David Cheifetz (link). Mine is a poor copy of Cheifetz's examples.
Mom, Wife, Former Pediatrician, One who LOVES color, creativity, paint, and lifelong learning.