Mesh Paint has two different modes: Color, which paints RGBA data directly onto a mesh's vertices, and Blend Weights, which serve as a means to blend between different textures. The default Mesh Paint mode is Color. You can select a paint and erase color and then apply those colors to the mesh using the brush. This mode is useful when your material is configured to combine the vertex color with your pixel shader in some interesting way.
When the painting mode is set to colors, you have these options available:
"Fills" the mesh or instance being painted using the Paint color, respecting the Channels settings.
Copies the vertex colors from the selected instance(s) to the source mesh asset.
Imports a .tga image file to use to populate the vertex colors of the selected instance(s).
The color that will be applied while painting (LMB + Drag). A swatch displays a preview of the current color. The color can be set using the Color Picker built-in to the tool.
The color to use as your "eraser" color while erasing (Shift + LMB + Drag). A swatch displays a preview of the current color. The color can be set using the Color Picker built-in to the tool.
Swaps the Paint color and Erase color.
These check boxes set which color/alpha channels should be affected by the paint brush.
This will locate the selected mesh within the Content Browser.
This will save any changes to vertex color that have been made to the asset. Note that this option is only available if the Paint option is set to Textures. If the option is set to Verticies, then the Save button will be disabled.
Once your color is selected, you only have to hold Ctrl and drag on a selected mesh to paint! Ctrl+Shift dragging will paint with the Erase color.