Mother attacking chicks


I have 3 chicks hatched by a broody.
They are now a week old and growing well, but as soon I let them out of the broody coop into the run, the hen starts showing them where food is etc and within 5 minutes is attacking the palest of the 3 chicks and throwing it in the air by its wing. This escalates until she is doing it to all 3. As soon as I pop them all back in the coop and it’s dark, she stops, settles down to brood and is again the perfect mum!?!
I have tried dying the pale chick purple so she looks as dark as the other two and I’m supervise them for the short periods they are in the run, but not sure it’s good to have them cooped up for so long in the dark and if she will ever stop attacking them?
I have raised numerous chicks with another broody over the years, but sadly lost her so this is a first time mum.
This is the first time I’ve seen anything like this though.
Anyone any advice or suggestions of what I can do to settle mum?
Thank you.