You can add a vertex color channel to your Stingray PBS ShaderFX network, then paint vertices.
In this example, we first created a Stingray PBS node, then connected a diffuse map and a normal map to its shader graph using the ShaderFX editor. See Create a Stingray Physically Based Shader and Build a Stingray Physically Based Shader using the ShaderFX editor, and Connect maps to a Stingray Physically Based Shader for detailed information on these tasks.
To add a vertex color channel to your Stingray PBS network
Your model appears black in the viewport.
This step is only required if you are painting on a object that has not previously had vertex colors applied.
You can now see the painted areas on your model in the viewport.
To add your vertex color channel with your base color map
The Add operation produces a float4 output (RGBA), but the Standard Base node Base Color attribute takes a float3 (xyz) as input. The float4 is converted to float3 automatically.
You can now see the object's color map and the areas you painted on your model in Viewport 2.0.