I don't recall her hair colour changing in The Abominable Bride, and I am also pretty sure that Louis Moffat as child Sherlock does not have red hair either.  So I'm not totally sure what you're getting at here. 

Maybe it is lighting.
I think Louis is quite dark, but not sure...
Dunno if they dyed his hair for the show.

Vhanja wrote:

I never saw anything red in Louis hair, it seemed dark brown to me. 

 
Thanks for the images, Vhanja. The second image is one that seems reddish to me, warmer than a pure brown. Auburn maybe. If I were to choose a paint color I'd definitely tone it with a translucent red oxide. But it's dark lighting, and the brown is there, too. The first image is tricky because it's such weird lighting, I think it would distort any color because of the blue tones in the light.

Regardless of whether or not Louis's hair is part of the equation, Molly's red hair (or the appearance of it) in TAB stood out to me like the red dress in "Schindler's List". Again, it could be down to lighting, but I do think it might be deliberate.

Obviously, one knows from prior observation that "Molly's" hair is not *actually* red, but as an artist and scientist I always try to work with what I see, and to ask questions when what I see doesn't add up to what I know. And from my own experience, I can suggest that disparities are often deliberate and meaningful. Hence, this thread. But maybe I just need to have my vision checked...

Haven't had a chance to read up on how to add images in a post. Will get there soon.

I think his hair looks brown in both pics, in the second one I think it's the lighting that makes it look auburn in the darkest parts. 

Molly's hair in TAB is trickier, it does seem to have a hint of red in it, but I'm not sure if that has any other meaning to it than pure esthetics. (Perhaps since she is only shown in scenes with little light a brighter colour would seem better than her "mouse brown" in modern day Sherlock? Dunno, just guessing).

lol Susi!

Also, the only thing I can think of as to why Molly's hair would look like a different colour in at least part of The Abominable Bride is because she's wearing a wig as "Dr. Hooper."