The Modulate Blend Mode is the default Blend Mode for the Light Painters. They will multiply the existing Scene Color with its own. This is useful in most Situations, but it can not be used in completely Dark Scenes.
The Additive Blend Mode will simply add the Light Color on top of the existing Scene Color. With this Blend Mode it can be possible to not see the actual Scene anymore.
(Deferred Renderer only, does not support Forward Shading or Mobile)
The Additive with Scene Color Blend Mode is the most expensive and most useful Blend Mode. It allows to light up completely Dark Scenes without using a default UE4 Skylight or Point Light.
It currently supports following Scene Textures: Base Color, Specular, Roughness