Modo 14.0v2 docs:

Using Painting Tools

Painting in Modo is an intuitive blend of 2D and 3D painting techniques, where you can paint onto any bitmap image map within any of the GL viewports.
The brushes for painting can be found in the Paint tab of the interface, where a focused collection of tools and useful commands are located dedicated to the painting and sculpting functionality. The tools may also be called from the menu bar, under the Texture > Paint Tools menu. Invoking any of the tools displays the associated properties in the appropriate viewport panel.

To paint, you need to select both a Tool (brush) and a Tip, which designates how the brush itself applies the color. When working with a mouse, you can click and drag over the surface to apply the stroke. The click is applied automatically when using a pressure sensitive stylus and tablet. You may also right-click and drag in the viewport to interactively scale the size of the brush tip.

Painting
	Airbrush	The Airbrush tool, with the proper Tip can be used to mimic a real airbrush which excels at creating soft, even strokes.
	Clone	The Clone tool provides a function to duplicate an area of a texture to another area.
	Radial Gradient	The Radial Gradient tool applies a flood fill of color that ramps from the FG color to the BG color in a circular fashion.
	Blur	The Blur tool averages pixels from under the brush to create the effect of softening any apparent detail.
	Paintbrush	The Paintbrush tool, combined with a Tip applies solid even strokes to the canvas.
	Line	The Line paint tool allows you to quickly create straight lines by clicking a start point, dragging to the end point position, and releasing the mouse button to set.
	Linear Gradient	The Linear Gradient paint tool applies a flood fill of ink that ramps from the FG color to the BG color.
	Sharpen	The Sharpen tool increases the edge contrast between colors producing an effect that looks like focusing, making images appear sharper.
	Eraser	The Eraser tool works by modifying the alpha channel of the currently selected image texture to create the effect of erasing the paint.
	Fill	The Fill tool colors all the pixels in the image that fall within the Fill Tolerance according to the color of the pixel that was clicked on when initiating the fill.
	Smudge	The Smudge tool shift pixels from under the brush in the direction of the mouse's or stylus' movement to create the visual effect of smearing the paint together.
	Lasso	The Lasso falloff option provides you a means to draw selection areas directly on a viewport that then limit the application of paint strokes to the surface.