In this test image, I adjusted Color Variability to paint with more than one color 
(Art Materials/Color Variability: Move the Hue slider to 25%). Some of the brush strokes were made with Impasto set to Color and Depth, Depth Method: Paper, some with Depth Method: Uniform, Depth: 300%, Smoothing: 32%, and Plow: 100%. 
The various strokes were made on separate Layers above a black Canvas. The final brush dab was made on the top Layer with the brush Size set to cover the Canvas. 

At the bottom of this page are some variations on this image, with the top Layer set to several other Composite Methods (Controls:Adjuster palette, Composite Method drop down list options). In the image above, the top Layer's Composite Method is Default.

This tutorial, another inspired by Athena in the Painter Forum at TalkGraphics, is written to show some of what can be done when a Captured Brush is created from a white drawing on a black Canvas.

1. Open a white Canvas and Fill it with black.

2. Lasso tool: Draw a freehand shape (I drew a "gingerbread man" of sorts). Your freehand shape is a Painter selection.

3. Edit menu, Clear (or Ctrl/Command+X) to clear the center of the selection. Now you have a white shape on the black Canvas.

4. Rectangular Selection tool: Hold down the Shift key and draw a square selection around your white shape. 

NOTE: If your square selection is not centered properly over the white shape, click and drag downward on the Layer Adjuster tool, stopping at the Selection Adjuster tool (hand with pointing index finger and dashed-line rectangle behind the hand), then click in the center of the selection and move it into position. When the selection is located where you want it, click and drag downward on the Selection Adjuster tool, stopping at the Layer Adjuster tool to make it the active tool again. 

5. Brushes palette: Pick the source brush type and variant (the brush on which your captured brush will be built). For instance, I chose the Pens brush Flat Color variant (because some brushes won't show when painted over a black background, like the Felt Pens Dirty Marker variant, for instance).

6. Brushes palette/Brush menu/Capture Brush.

7. Brushes palette/Variant menu/Save Variant. Give it a unique name not already used by Painter. For instance, "Jin's GingerbreadManPensFltClr". (That's a pretty long name and since there's a limit on the number of letters allowed, I had to abbreviate and leave out spaces. However, it will help me remember that the source brush variant was Pens Flat Color.)

NOTE: The source brush variant (Pens brush, Flat Color variant) was not altered permanently because the new captured brush variant was saved with a unique name not already used by Painter). To easily restore the source brush to its default state first click the appropriate brush variant (Pens brush, Flat Color variant), then do the following: 

Brushes palette/Variant menu/Restore Default Variant

The new brush variant is now listed in the Pens Brush Variant Popup list. To move the new custom variant to a new brush category:

1. Brushes palette/Brush Menu/New Brush: Give the new brush category a unique name, not already used by Painter (for instance, "Jin's Brushes")

2. Brushes palette/Pens brush: Click the new captured brush variant ("Jin's GingerbreadManPensFltClr").

3. Brushes palette/Variant menu/Copy Variant: Scroll down the list of brush categories and pick the new brush category, "Jin's Brushes", then click the OK button.

4. Brushes palette/Pens brush: Click the new brush variant "Jin's GingerbreadManPensFltClr" that's still listed in the Pens brush variants list (remember, you have a copy of it in the new brush category ("Jin's Brushes") so it's OK to delete this one.

5. Brushes palette/Variant menu/Delete Variant.

Now the new captured brush ("Jin's GingerbreadManPensFltClr") is listed as a variant in the new brush category ("Jin's Brushes") and it's been deleted from the Pens brush category's variants. 

Below are some variations on the image you saw at the top of the page. Under each is the Composite Method applied to the top Layer (the one with the large brush dab):
 

Difference (Click to compare with Default)	Luminance (Click to compare with Default)
Hard Light (Click to compare with Default)	Soft Light (Click to compare with Default)
Pseudocolor (Click to compare with Default)	Reverse-Out (Click to compare with Default)