Now that I have rechecked into this I see how I misinterpreted what I was
seeing. What I had thought was a "artifact" must be a place where the
negatively colored light source has its own shadow region, thus not
counteracting the positive light at that place. I figure I better post this
anyway to show others how this kind of thing looks and acts.

Top rendering shows the everyday kind of spotlight (rgb +1) pointed downward
but pretty much at full wide angle (the orientation is a little off from the
y plane). Shadow of the sphere appears as it should.
Middle render is the negative color light (rgb -1) also pointed downward but
shifted slightly lower than where the other is placed. Looks okay again. The
ambient light is from a dim shadowless light source.
Bottom render shows the interactions of both lights within a shadow region
of the sphere. The negative light, having a shadow of sorts too, doesn't
fully match up with the positive light's shadow so they are offset some. I
had thought this was wrong before since I expected no light to be anywhere
the negative light source covered, even in shadow. In other words, negative
lights do have shadows.

Sorry about the confusion.
--
Farewell,
Bob

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