Give it too much sun while you're actually applying the suspension, and it'll go too dark.
Ideal for the brush or pinch method is a open to skylight but not sunlight. You can use sunlight to dry after the rinses, but not while actually applying.
Temperature should be not too hot, not too cold. Too hot causes to dry too quickly, too cool causes to dry too slowly and you often finish up with a greeny-yellowy mess. 25C-30C sounds good.
Same with humidity:- too humid it dries to slowly, too dry it dries too quickly.
I've just said that direct sunlight will cause it to go too dark, but for somebody with a lot of experience, hot direct sunlight can be useful, provided you're prepared to do it again and again and again and clean off between stains. You need the experience to give it just the lightest of light touches when it is approaching excellent. Probably the best stain jobs I've ever done have been done in direct sunlight, but I honestly cannot recommend that everybody do it this way, because it is far, far too hard to control.
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