In making web content accessible to people with varying levels of vision impairment, attention is often focused on providing alternatives for conveying the meaning of images and ensuring that text can be properly conveyed by screen readers.
It is also necessary to pay attention to how reliant the communication of the content is on other visual cues, including size and shape. This is addressed in Success Criterion 1.3.3: Sensory characteristics:
Instructions provided for understanding and operating content do not rely solely on sensory characteristics of components such as shape, size, visual location, orientation, or sound. (Level A)
People with some kinds of disabilities have limitations on their ability to distinguish or apply meaning to the characteristics of elements that rely on sensory perception.
However, Success Criterion 1.3.3 does not deal with colour. The use of colour in conveying meaning is covered in Guideline 1.4. You can find more information in the topic on colour for content authors.
Consider the following instruction: "Press the round button to submit the form, or the square button to cancel and clear the form."
This presents a real problem for a person who cannot distinguish round from square, including people with a range of specific types of visual impairment. Even a screen reader may have trouble conveying the distinction unless properly informed.
Yet this type of instruction is not uncommon in some of the most critical processes that a user can face on a webpage: the submission of user information to complete a procedure that may have great implications, including financial.
It's also important to be aware that the use of different shapes, changes in sizes, location on the webpage, which way round things are and auditory signals like beeps are known to be useful in conveying web content to people with some kinds of cognitive, learning and intellectual disabilities.
There is no need to refrain from using sensory characteristics of objects and elements to convey meaning. The key lies in the use of the word "solely".