Why is color contrast important in Outlook?
Having a high level of contrast between foreground and background results in more people being able to see and use the content. Color and hue are not as important as high visual color contrast between the background and text to those with low vision.
It is good practice to use bright colors or high-contrast color schemes on opposite ends of the color spectrum. White and black schemes make it easier for people who are colorblind to distinguish text and shapes.