Many insects have nano-scale layers in their wings and bodies that refract and reflect iridescent light. You will observe vivid colors that change color as you move your viewing angle or the light source. Areas that do not reflect iridescent light drop suddenly to black with little transition. You can simulate these effects in your drawings by placing vivid colors adjacent to each other and rapidly transitioning to black.
As in other skills, learning how to draw insects takes practice. Start by looking at photographs of iridescent insects online and practice with these. I saw a beautiful photo on BugGuide.net by Andrew Mckorney and he gave me permission to use his photo for this demonstration. Thank you Andrew! You can print a beetle worksheet (three line-drawings of the beetle in this demonstration) on a piece of mid-weight watercolor paper and use it to experiment with and practice drawing iridescence. Click on the first image to enlarge it and follow the step by step demonstration.
I began with a line drawing based on the photograph in BugGuide.net.
I painted highlight areas with bright cyan colored paint (Phthalo Blue).
I mixed violet (Phthalo Blue & Quinacridone Pink) over the cyan base.
Once the bright areas were dry, I overpainted with a heavy coat of blue-black (Payne’s Gray).
If you are painting on heavy watercolor paper, you can actually remove colors by rewetting parts of the painting and lifting color out with a damp brush or tissue. This does not work on lightweight sketchbook paper. Using this technique I softened the edges of some of the dark areas and lifted out some of the dark areas.
Lifting out the darks also lightened some of the other colors so I added more violet on parts of the body. You can add layers and layers of paint without smearing the underlying layers if you let the paint dry between coats.
I added crisp details with a black pen. Add details at the end of the drawing. If you put them on at the start, you can lose or smear them as you play with layers of paint.
Add final highlights with a light blue Prismacolor pencil. be careful not to overdo it. This part is really fun and it is hard to know when to stop. Note how I added little flicks of color at the edge of the surface pits that are on the side of the pit furthest from the light source.