It’s a little bit out of the scope for us to show you how to add a colour picker to your application and change the colour of BoxAnnotation.
However, we do have a similar example here showing a colour picker to change series color.
What you need to do is:
Further resources can be found online, such as https://stackoverflow.com/questions/17089382/wpf-color-picker-implementation and https://www.c-sharpcorner.com/UploadFile/mahesh/wpf-colorpicker/
Please login first to submit.