Choice-1.py

class Car(models.Model):
    COLOR_CHOICES = (
        ('RED', 'red'),
        ('WHITE', 'white'),
        ('BLUE', 'blue'),
    )

    color = models.CharField(max_length=5, choices=COLOR_CHOICES, default='RED')
    # ...

red_cars = Car.objects.filter(color='RED')

Choice-2.py

class Car(models.Model):
    RED = 'RED'
    WHITE = 'WHITE'
    BLUE = 'BLUE'
    COLOR_CHOICES = (
        (RED, 'red'),
        (WHITE, 'white'),
        (BLUE, 'blue'),
    )

    color = models.CharField(max_length=5, choices=COLOR_CHOICES, default=RED)
    # ...

red_cars = Car.objects.filter(color=Car.RED)

Choice-3.py

from enum import Enum

class Color(Enum):
    RED = 'red'
    WHITE = 'white'
    BLUE = 'blue'

# They're iterables
print([c for c in Color])  
# >>> [Color.RED, Color.WHITE, Color.BLUE]

# Comparing..
Color.RED == Color.RED  
# >>> True

# but comparisons against non-enumeration values will always be false.
# (See IntNum if that's something you're interested in.)
Color.RED == 'red'  
# >>> False

# Values are accessed through .value.
Color.RED.value == 'red'  
# >>> True

color = Color.RED
color.name
# >>> 'RED'
color.value
# >>> 'red'