Lighting conditions matter. While not as sensitive as the dried "emulsion", the mix is sensitive to light. The raw chemicals (ammonium ferricitrate and silver nitrate) are sensitive as well, and should be stored dark.
I wouldn't worry about incandescent lights while mixing, but fluorescent or daylight would be a bad idea. Subdued light shouldn't be necessary, except if the mixing takes a very long time.
(Yet another reason why I love my stirrer - I start mixing, then leave the room and check it every once in a while if it takes a long time to dissolve)