Some clouds, it turns out, contain large populations of bacteria. There is some
complex ecology going on up there, because the bacteria act as nucleation sites for
water droplets, and can also influence the freezing of those droplets. Thus, these
bacteria have impacts on rainfall and snowfall.
It's very likely, in fact, that the
bacteria impact clouds in a way that is beneficial to the bacteria.
Now, this is all very interesting and eco and gaia and all that, but who (frankly) gives a
flying fuck? Well, technically the bacteria don't - although some of them do have the
equivalent of sexual reproduction.
Howevertheless, the presence of cloudborne bacteria open up unlimitless possibilities.
In particular, some bacteria are bioluminescent, and others can generally be
engineered to be so. Note that I'm not talking about fluorescence (a la GFP) here, but
actual bioluminescence. With a bit of tinkering we can get most colours from red
through to greeny-blue.
The first step in the process is to grow up shedloads of these bacteria, in a chosen
colour, in a regular lab fermenter. We're talking a few hundred litres of dense culture,
amounting to maybe 10^15 bacteria.
The second step is to fly through some nice low clouds as evening approaches, spraying
these bacteria as a fine mist.
The third step is the clever one: there is no third step. We just sit back and enjoy the
ghostly green glow of a cloud, perhaps swirling and mixing with the red glow of another
cloud, and finally dissipating as the bacteria get out-competed by their sleeker, better-
adapted non-luminous cousins.