CKEditor: Is it possible to limit the color choices for TextColor and BGColor?

Yes, this is easy to do:

in your settings.py, you have

CKEDITOR_CONFIGS = {
     'default':{
        toolbar': 'default',
        'colorButton_colors': '008000,454545,FFF', #to set specific colors
        'colorButton_enableMore': False, # to remove more color button
},

Note these two lines are what you need to add.

        'colorButton_colors': '008000,454545,FFF', #to set specific colors
        'colorButton_enableMore': False, # to remove more color button

The widely used django-ckeditor is based on ckeditor4. If you want to modify default behavior, you search for ckeditor 4 doc.

For example, you are interested in text and background color, you google search ckeditor4 text background color, doc about text color will show up on your search result. There you see:

# this is a javascript code, you need to convert to python code.

config.colorButton_colors = 'CF5D4E,454545,FFF,DDD,CCEAEE,66AB16';

#change to :

'colorButton_colors': 'CF5D4E,454545,FFF,DDD,CCEAEE,66AB16', 

#and put into CKEDITOR_CONFIGS in settings.py