A big part of writing Alt-text is being as direct and concise as possible. The words we use have a big role in how this is accomplished. You can learn more about being direct and concise on the General Guidelines page.
Remember, the most direct way to describe something is to stick to the facts and describe what it looks like. If there is an object that is abstract, futuristic, or complex in its shape you can describe it is relation to another common object. As long as it invokes the right imagery in your mind.
On this page you will find a list of terms to use for describing trickier parts of images, including, but not limited to, facial expressions, culturally specific clothing, buildings, etc..
In this section you will find options for descriptive words. This is an ongoing list that will be added to and adapted.
For images in Black and White see Describing Colour in Images
Most colours you simply state what it is:
For descriptors of colours, you want to stick to the facts and be objective. Some context requires more emotive descriptors, and this is based on the surrounding text and audience. When in doubt, post the picture and your question on the Alt-text Q&A
Some common descriptors include:
You don't always have to describe in detail the pattern of someone's shirt, and can often just state something like: floral pattern or checkered or plaid. If the context calls for more details, stick to the facts. The following words can help you. You can also provide an example of what a texture may be like, if it is clear in the context, for example: smooth as glass.