The Tint tool allows you to paint onto strokes point mixing the material base color with a selected vertex color.
Selecting a Brush, Color & Mode
In the Tool Settings select the brush, color and mode to use with the tool.
You can configure the brush main settings included in the Tool Settings for convenience. For the vertex paint brushes configuration and settings see Vertex Paint Brush.
Ctrl-LMB erase the vertex color.
Click and hold LMB or use the pen tip to paint onto the stroke points.