How to Paint a Rainbow in Watercolor

Where are a few details of rainbow optics that many artists miss. Look for these and your rainbow sketches will be more accurate and you will also learn to see more in the sky. First a few observations:

The sky is lighter inside the rainbow. This is due to light that is reflected back to the observer in all of the wavelengths instead of selected colors as at the rim of the rainbow.
We all know that there spectrum is Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROY G BIV or Richard Of York Gave Battle In Vain). However many are not aware that the bands are not of equal width. Look at the image of the spectrum (taken from NOAA’s website). Note the predominance of the orange, green and blue light and that there is no clear yellow band.
The red is at the top of the rainbow.
If there is a double rainbow, there is a dark band between the two and the outer rainbow is both lighter and reversed (so that the two red bands face each other).
The rainbow’s shape will be some part of the upper half of a circle. If you see a rainbow from an airplane, you might get to see a ring, but on the ground it is almost always less than a half a circle, getting higher in the sky (more of the circle) toward sunrise and sunset.
When you see a rainbow, the sun is directly behind you. Other shadows in the drawing must correspond to the same sun position. This is easy if you are drawing from life but a mistake often made in imaginative drawing.

Here is a demonstration of a rainbow over a marsh as the storm clears. Click on the first image to start a step-by-step annotated slide show.

Start with a line drawing of the landscape. In this drawing note how the lines in the middle and background converge toward the center. This is a drawing trick you can use to suggest open spaces and flat planes.

Paint the sky with a heavy wash of gray and while it is still wet, add darker clouds at the top with the stroke and bleed effect described in the previous post.

Let the sky dry and add a last coat of gray outside the arc of your rainbow.

Underpaint the rainbow with permanent white gouache. Paint the marsh with a dull brown-green mixture, brightening to warm green in the foreground (warm colors tend to feel close to the observer).

Add a dark strip between the water and the marsh vegetation and a reflection in the water. Note that you can leave the water white and it still looks like water.