Ideas:

1. Yesterday a new pope was elected. At first the smoke seemed dark, though. When the straw is wet it gives dark smoke, otherwise white. Wednesday is the day of water. But yesterday it was Tuesday.

2. Wednesday is in the middle of the week. If the week is similar to the cosmos, then Sunday + Monday represents the sky (light) while Friday + Saturday represents the underworld (dark). In between we find our home, the earth, which has both light and shadow (Tuesday + Wednesday + Thursday).

3. In the middle of this 'earth' there is Wednesday and its colour ought to be blue-green as water.