The difference between text and its background color (or between different parts of an image) is called contrast ratio. A high contrast ratio makes the slides easy to read, especially when people are sitting far away or with low vision. For example, pie charts often have different colors for each portion of the pie. Plain black or white text may not offer enough contrast for some colors. At a minimum, use a 3:1 contrast ratio for large text and 4.5:1 for other text and images. An ideal contrast ratio is 7:1. To check contrast, use the WebAIM contrast checker.