kivy: color[3] value ignored

The alpha channel isn’t respected as it should

This label should be gray, but is bright white:

#!/usr/bin/env python2
# -*- coding: utf-8 -*-
from kivy.app import App
from kivy.lang import Builder

x = Builder.load_string('''
Label:
    text: 'test'
    color: 1, 1, 1, .3
''')


class Test(App):
    def build(self):
        return x

Test().run()