The grey textures are labelled with percentage values which indicate how well they reflect light. The higher numbers are lighter shades, and the lower numbers are darker shades. Many people on this forum prefer reflectivity 50% because it is easy to look at and provides good contrast with the tf2 models. However, others use darker shades in their map and there is nothing wrong with that, either. It's a personal choice and you might start to like using one over another when you block out maps.