PyMOL ray_trace_mode 1: the gain is white and not black

Question

[anders.brakestad]

I want to make a figure of an enzyme structure with PyMOL's ray tracing feature. I set the ray trace mode to mode 1

set ray_trace_mode, 1

and I trace the image

ray 1000,1000

However, the resulting image has the "gain" as white, and not black. Why is that? I see no option for changing this setting. Does anyone know how to change this?

I am using PyMOL 2.0.4 for Mac (Educational version).