There is a lot of "gray area" in this prompt. For example, your writing is going to carry a lot more or less detail depending on if the response is a paragraph or an essay . My initial thoughts are that the prompt is asking for something similar to a...
There is a lot of "gray area" in this prompt. For example, your writing is going to carry a lot more or less detail depending on if the response is a paragraph or an essay. My initial thoughts are that the prompt is asking for something similar to a descriptive essay; however, a descriptive essay usually focuses on describing things that target all five of a reader's senses. This prompt indicates that it is specifically inquiring about sight.
Due to the fact that the description is limited to sight, your response should definitely focus on color. I'm quite certain that "gray" comes to mind, but that is hardly descriptive. What kind of gray? It is navy destroyer gray? Is it African elephant gray? Give your readers something concrete to compare the color to, or give your readers a simile, like "the sky was as gray as newly mixed concrete."
You should also picture what kind of clouds are making the rain. If the clouds are stratus clouds, the clouds generally cover the sky as far as the eye can see, and they are layered up layer after dismal layer. These clouds don't usually bring driving rain. They bring light showers for hours. That is a very different kind of rain than rain that comes from a cumulonimbus cloud. Those storm clouds bring massive amounts of rain in a very short amount of time, and the rain is often accompanied with lightning. The flashes of lightning could add an entirely new description of color to your piece as they flash a white-blue across the sky and turn gray into moments of pure white.
Your writing could also explore how the rain makes the world look empty. It's an odd thing because your vision is taking in all kinds of light reflecting off of millions of drops of water in the air, yet the world feels empty because far fewer people are out and about in a rain storm.
Write about the rivers of runoff that are flowing down various driveways, down curbs, and into storm drains. Write about how the water that covers the streets gives them the appearance of having an icy covering. Write about the random rays of sunlight that occasionally break through random openings in the clouds. Write it all out in a stream of consciousness the first time, then rewrite your thoughts to better organize them into specific areas of focus (clouds, sky, streets, etc.) for your reader.