Purple People Eater

I had a happy lighting accident this evening:

RAAA

T-Rex is standing on a mirror, in front of a white background. I fired a flash with red “gel” (actually a silk handkerchief from my old magic gear) into the mirror at 40ish degrees to my left. I expected the red filter to give a bloody, macabre feel, and the resulting pink/purple gradient took me by surprise. I love the effect, but I’m puzzled – surely it should go from bright red –> dark red –> black? I’m wondering whether it’s a white balance thing, although I can’t see how that makes sense…

I hope to read this post in a few months and think “that’s so obvious”.