Fiery clouds like these ? Since it is not possible to illuminate spectral clouds from the inside (only sunlight affects them), I had to use the same trick used to make trees covered with snow. Essentially, make a copy of the clouds, change the color and move them slightly above or under the original clouds. Here, you have one group of clouds in Black and the same group in Red, slightly moved under and away from the Black group to give the illusion of clouds illuminated from under. It's not perfect but it kind of works.
