The following is my opinion on how to draw a rainbow simply:
Preparation materials: drawing paper, brush, watercolor paint (or colored pen).
Draw a circle: first draw a circle on the canvas as the outline of the rainbow. Different colors can be used to increase layering and beauty.
Add color: Next, choose your favorite color, fill the circle with color, and fill it with red, orange, yellow, green, blue, indigo and purple pigments (or colored pens) in turn. You can use the gradient effect to simulate the color change of a rainbow.
Add light and shadow effects: In order to make the rainbow more realistic, you can add light and shadow effects around the circle. Draw light around the circle with white or light color to simulate the effect of sunlight.
Perfect details: Finally, you can add some details to the rainbow with fine brushstrokes, such as lines, spots or textures, to increase its complexity and beauty. Draw the Chu-Qing background after the rainbow rains, such as blue sky, distant mountains, near grass and so on. To increase the layering of the picture. You can add some highlights and shadows to the rainbow with a small brush to make it look more vivid.
In a word, drawing a rainbow requires some patience and care, but as long as you follow the steps, you can easily draw a beautiful rainbow.