Rain is tricky subject matter regardless of the medium one is using. It can't be truly seen at a distance, and even up close the eye doesn't focus on any one drop. It is a subject that is visible and invisible at the same time. I have a few methods for achieving the impression of falling rain in watercolor.
Technique 1: Wet on Wet
Wet on wet is a classic watercolor technique. For achieving the effect of distant rain it is quite useful.
Wet on wet is simply a technique in watercolor is when wet watercolor pigment is applied to wet paper. A layer of pure water is applied to the paper, followed by a wash of color.
Prep: Using an artists tape, secure your watercolor paper to a stiff surface you are able to move(I recommend a piece of cardboard). Have ready a cup of clean water, as well as your chosen pigment dissolved in a well of water in your palette. Finally you will need a soft watercolor brush(a medium to large round will work best, but many sizes of soft brush will do).
To use this technique for rain wet your paper with clear water(Note, do not spread the water to any places you do not want your pigment to travel).
Once your water is down, take a brush loaded with your chosen pigment and apply it to the top of the page. Tilt the paper in the direction you want your rain to travel, this could be straight down or at an angle(to imply wind).
This method for creating rain is best for distant rain. It gives the impression of the weather. Mimicking the visual of rain clouds at a distance(see below example).
Technique 2: Lifting
Lifting is another useful technique in watercolor. And it can be used to give the impression of falling rain.
Lifting is when watercolor pigment is lifted off of the page either using a dry, or clean wet brush. This can be done when the pigment is wet or dry.
Prep: Follow the above instructions to prepare your paper and materials. The only additional tool you will need is a fine liner brush.
This process requires two steps. First is a wash of whatever color you would like for your rainstorm. Use wet on wet to apply clean water to your watercolor paper, then lay down a wash of pigment. This pigment needs to be fairly saturated for this technique to work.
Allow the painting to dry completely.
Using your fine liner brush apply a thin short line of water to the dry pigmented area of your painting. Clean the brush quickly, then lift the water and some of the pigment along with it. The result should be a lighter colored line in your pigment. Please see my video here for a full visual example. Continue making these thin lines, in the same direction on your painting until you have enough. This will give the effect of falling rain(see examples below).
Here are both completed example pieces
Dark colors work best with both of these techniques. You can get creative and use some non-traditional colors, but I recommend sticking to dark/cool colors for the best results. In most of my work depicting rain I like to use Payne's Gray.
Colors like red orange and yellow might work with the first wet on wet technique, but would certainly not work well at all for the lifting technique. Blues, purples, grays, blacks, are your best bet for good results.
Also note that for both techniques you want a fair amount of saturation in your pigment to start with. Both of these techniques use wet on wet, which will further dilute and thin your pigment, so it's good idea to start with more as that will likely not be the pigment concentration you end with in your painting.
Here are some more examples of my work where I have used the above techniques.
This lesson is also available on my Youtube channel. There you will be able to see the painting process.
These techniques will only work in watercolor, and perhaps gouache. Painting rain is tricky, and presents different challenges depending on the medium.
Now go forth and paint the rain!