Broody hens do not look at babies and see different colours or sizes, I've heard stories of broody hens not only mothering chicks but of mothering kittens too! Once their hormones are surging, they will mother anything!!
I've smuggled chicks under broody hens before and it's actually relatively easy. It works the best when the broody hen has been broody for at least a couple of weeks.
Rule #1 - Always do the deed at night, in the dark, when she's calm and asleep.
Rule #2 - Its best to do it from behind, so she doesn't see them first.
Rule #3 - Do it as quickly and safely as you can, with as little disturbance as you can manage.
Rule #4 - keep light and noise to a minimum.
Once you've got them under her, leave and shut the door and wait outside and listen, what you are listening for is .... silence! If it is all quiet after you leave, all is good and she has accepted them and will wake in the morning a happy and fulfilled mumma. Check again an hour or so later and voila!! Success.
Just be prepared, in case things don't go smoothly and you may have to remove them. But I think it's unlikely, a broody hens goal is to hatch chicks and I don't think she cares how, when she feels them under her, she will be content.
Good luck
