You can technically use any brush, but for best results, it’s highly recommended to use brushes made specifically for watercolor—and here’s why:
Watercolor brushes are designed to hold water in their bristles. This helps you make smooth, flowing strokes—something regular brushes often can’t do.
Watercolor brushes usually have a soft, springy tip that gives you more control for delicate lines or wide washes.
Other types of brushes (like for acrylic or oil) are stiffer and can scratch or damage the delicate watercolor paper.
You can experiment with other brushes if you're just playing around, but if you're serious about watercolor, investing in a few proper watercolor brushes will make painting easier and more enjoyable.