PS brushes are not colored. There are two situations:

First, the foreground color is not set. Solution:

1, click the foreground color.

2. Select a color in the color picker, as shown below:

Second, if you can't choose a color, it means that the color mode of the document is gray, and the solution is:

1. Click Image Mode in the main menu and select RGB mode, as shown in the following figure: