How do I color-code grades on report cards?

On the Settings page, open the "Report Card" section. Here you will find three separate settings that allow you show grades in color for A) assessment results B) course results and C) standard results. Each is set independently so you can use none of them, all of them, or just one or two of them.

Note that – by default – colors are automatically assigned to grading scale levels from red (lowest level) to dark green (highest level). If you want to define your own colors, check out this article.