A Brush is not a primitive type, it's used by Shape2D based primitive to define how the shape's border and/or fill content should be drawn.
There are actually two types of brush SolidColor and Gradient. You can access a brush from the Canvas class, using these static methods:
GetSolidColorBrush(), returns a Brush of the given color (using a Color4 object).GetSolidColorBrushFromHex(), returns a Brush of the given color using a CSS style string, like "#FF0000FF" for plain red, "#FF000080" for a half transparent red.GetGradientColorBrush(), returns a Brush using the given gradient value (color1, color2, translation, rotation).