Why does the form with the SelectMultiple widget not work?

Question

Django

I have implemented a form on a page built on a model.

My forms.py file:

class DogRequestsForm(ModelForm):
    class Meta:
        model = DogRequest
        fields = ['id_name', 'Color', 'Dog']
        widgets = {
            'id_name': NumberInput(attrs={
                'class': 'form-control',
                'placeholder': 'id_name'
            }),
            'Color': SelectMultiple(attrs={
                'class': 'form-control',
                'placeholder': 'Color'
            }),
            'Dog': SelectMultiple(attrs={
                'class': 'form-control',
                'placeholder': 'Dog'
            }),
        }

my models.py file:

class DogRequest(models.Model):
    id_name=models.PositiveSmallIntegerField()
    Colors=models.ForeignKey('Color', on_delete=models.CASCADE, blank=True)
    Dogs=models.ManyToManyField('Dog')

According to the django documentation, the ForeignKey and ManyToManyField forms use the SelectMultiple widget that I specified in my forms.py (for dog and color), but when I save the data in the form, I get an error about incorrect filling.
Moreover, if you remove these widgets for these fields, saving works correctly. What is wrong in my code?