Introduction
Stroke with original base material color (left) and with vertex painting (right).
When a point is painted, the color of the points is mixing with the base material color according to the settings of the brush.
Note
A vertex in Grease Pencil is called point. Point and vertex names are equivalent.
3D Viewport Mode selector set to Vertex Paint Mode.
Vertex Paint Options
General Vertex Paint options.
Selection Mask
You can use the selection tools in the Toolbar for a quick selections.
You can restrict painting only on the selected points or strokes with the Selection mode toggle.
Multiframe
Sometimes you may need to modify several frames at the same time with the painting tools.
You can activate multiframe edition with the Multiframe button next to the modes selector (faded lines icon). See for more information.