You are not at the right place, this is your LightAccumulator.
I guess you want to have a shadow color, so it depends what you are doing in your BxDF function, called from
FDirectLighting IntegrateBxDF( FGBufferData GBuffer, half3 N, half3 V, half3 L, float Falloff, half NoL, FAreaLight AreaLight, FShadowTerms Shadow )
There you should define your shadow color or get it from custom data in GBuffer.
Afterwards you set diffuse to a lerp value between ShadowColor and BaseColor depending on NoL.