What is low contrast?
We can use our feelings to know the criterion of "low contrast." But some people have good eyes and a lack of knowledge, and probably lack empathy.
Low contrast means any combination of two colors that has a ratio of less than 4.5:1. The ratio comes from the Web Content Accessibility Guidelines (WCAG) version 2.1, Success Criterion (SC) 1.4.3 Contrast (Minimum) (Level AA). I often use Colorable to check the contrast ratio between two colors. Use it—or any other tools—to make sure your content has enough contrast.
But that ratio is still too low for me. I believe many people think the same. I am comfortable with at least 9:1 ratio and above.
What is okay to be low contrast?
Decorative things such as background patterns or any decorative styling. One of the decorative styles is the background color of a section. It does not need to have a good color contrast ratio with the page background color.
What is bad about low color contrast?
Your text will be hard to read, if not unreadable. People will have a hard time. They will just get out of your website. But if you are a developer of a government website, you make them suffer because they have no other choice. Your images may not be clear to your users. If you make a custom image with a design tool, make sure they have enough contrast. Otherwise, you are saying to people that you want them to read, but actually you do not.