What is Contrast?
Contrast is the difference between the text color and the background color, measured as a luminance ratio. The accessibility reference is WCAG: at least 4.5 to 1 for regular text and 3 to 1 for large text. Below that, the slide looks good on a monitor and is unreadable on a phone with low brightness or in the sun. The common traps are light text over a light photo, brand color over brand color and gray over white. The fix is usually a darkened background behind the text, a colored box or simply changing the font color.