Text that is offset poorly by its background can be difficult for people with poor vision to read. This can also apply to icons or images on a page.
The following example shows two color combinations: one that has a low contrast ratio and one that has a high contrast ratio.
Color contrast checking tools can compare two color values and report on the contrast ratio between them. Some tools will allow you to adjust these values until the ratio is sufficient, helping you to choose a color scheme that avoids contrast problems.
Check text and background color combinations on the page, using a color contrast checking tool: