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.