Accessibility is the extent of which a product is accessible for everyone, regardless of their level of physical or mental ability. That's why barriers, that make products difficult or impossible for people to use, should be avoided. In digital design you can for example think of screen readers or voice control for blind people, but also think of solutions for people with less visible disabilities like colour blindness. For them design that uses only colour to convey information is useless. Designing with all kind of disabilities in mind helps to establish great accessibility.
We should also add a label instead of only colour to improve the accessibility of the website