In CSS, you can control the color of an element's border using the `border-color` property. This property allows you to set the color of the border around an HTML element. Here's how you can use it:
```css
.element {
border-color: #FF0000; /* You can use different color values */
}
```
In this example, the `.element` will have a border with a color of red (`#FF0000`). You can use various color representations, such as hexadecimal (`#RRGGBB`), RGB (`rgb(255, 0, 0)`), or color names (`red`, `blue`, etc.).
If you want to set different border colors for different sides of an element (top, right, bottom, left), you can use the `border-color` property with individual values:
```css
.element {
border-color: red green blue yellow; /* top right bottom left */
}
```
This sets the top border color to red, right border color to green, bottom border color to blue, and left border color to yellow.
Remember, the `border-color` property is part of the CSS border styling options that allow you to control the appearance of borders around HTML elements.
