[Django] Changing field widget when when using inlines

admin.py

from django import forms
from django.contrib import admin
from django.contrib.sites.models import Site
from django.contrib.sites.admin import SiteAdmin
from django.utils.translation import ugettext_lazy as _

from colorful.fields import RGBColorField  # third-party app

from .models import SiteOption


OPTION_COLOR_VARIABLE_CHOICES = (
    ('header_color', _('Header Color')),
    # ...
)

OPTION_VARIABLE_CHOICES = (
    ('contact_email', _(u'admin@site.com')),
    # ...
)

# Register site option model as inline into sites
# by unregistering Site model first
class ThemeColorsInlineAdmin(admin.TabularInline):
    model = SiteOption
    verbose_name = _('Theme Color')
    verbose_name_plural = _('Theme Colors')
    extra = 0

    def formfield_for_dbfield(self, db_field, **kwargs):
        if db_field.attname == 'variable':
            # Change variable field's widget with selectbox!
            kwargs['widget'] = forms.Select(
                choices=OPTION_COLOR_VARIABLE_CHOICES)
        if db_field.attname == 'value':
            # change value field's widget with color picker!
            db_field = RGBColorField()
        return super(ThemeColorsInlineAdmin, self).formfield_for_dbfield(
            db_field, **kwargs)

    def get_queryset(self, request):
        """
        Limit result to get only color variables.
        """
        qs = super(ThemeColorsInlineAdmin, self).get_queryset(request)
        return qs.filter(variable__in=[f[0] for f in OPTION_COLOR_VARIABLE_CHOICES])


class SiteOptionInlineAdmin(admin.TabularInline):
    model = SiteOption
    extra = 0

    def formfield_for_dbfield(self, db_field, **kwargs):
        if db_field.attname == 'variable':
            # Change variable field's widget with selectbox!
            kwargs['widget'] = forms.Select(choices=OPTION_VARIABLE_CHOICES)
        return super(SiteOptionInlineAdmin, self).formfield_for_dbfield(db_field, **kwargs)

    def get_queryset(self, request):
        """
        Limit result to get only non-color variables.
        """
        qs = super(SiteOptionInlineAdmin, self).get_queryset(request)
        return qs.exclude(variable__in=[f[0] for f in OPTION_COLOR_VARIABLE_CHOICES])


class MySiteAdmin(SiteAdmin):
    inlines = [ThemeColorsInlineAdmin, SiteOptionInlineAdmin]

admin.site.unregister(Site)
admin.site.register(Site, MySiteAdmin)

models.py

from django.db import models
from django.contrib.sites.models import Site
from django.contrib.sites.managers import CurrentSiteManager


class SiteOption(models.Model):
    variable = models.CharField(max_length=80)
    value = models.TextField()
    site = models.ForeignKey(Site)
    objects = models.Manager()
    on_site = CurrentSiteManager()

    class Meta:
        unique_together = (('variable', 'site'),)