Here is the scene rendered with standard C4D textures. Not very appealing is it.
Below you will see the setup a standard C4D material using the standard renderer: colour goes in the Color section, Specularity in the Reflectance section, etc.
Here you can see that the detailed surface colour information is contained in the Color channel. Here is the same texture reworked for the reflectance model of rendering:
The texture information that was in the Color channel has now been copied over, and placed in the Diffuse layer of the Reflectance channel. The Color channel has been disabled. All other aspects of the texture remain unchanged. Once I had changed the project over from the Standard renderer to the Physical renderer, I simple went through each texture setup, and copied or moved the texture information that was in the Color channel over to the Diffuse layer in the Reflectance channel.
Thank you for reading, and I hope that this tutorial was helpful.