The vray override material is a great way of previewing/checking your lighting even when you are a long way into a project. The scene below uses a homemade dusk HDR skydome and vrayIES internal lights which have a colour temperature of 4500 (6000 in the second image) kelvin. I wanted to check the effect 2 alternative IES web files made to the internal lighting in distribution as well as the fancy patterns on the walls. I find having all the materials a neutral colour really helps in balancing natural & artificial light.
To set it up, you first make a mid grey material in the material editor and then drag it into the vray override mtl slot in global switches (in this instance I made a material called '200' which has the rgb values 200,200,200):
then you can specify which objects to exclude from the override material (normally glass) in the include/exclude dialog: