just use equals, for example:
class MyModel(db.Model):
colors = db.StringListProperty()
MyModel(colors=['red', 'blue']).put()
MyModel(colors=['green', 'blue']).put()
MyModel(colors=['red', 'green']).put()
color = 'red'
query = MyModel.gql('WHERE colors = :1', color)
models = query.fetch(10)
assert len(models) == 2